Monolithic piezoelectric sensor (MPS) for sensing chemical, biochemical and physical measurands

A piezoelectric sensor and assembly for measuring chemical, biochemical and physical measurands is disclosed. The piezoelectric sensor comprises a piezoelectric material, preferably a crystal, a common metal layer attached to the top surface of the piezoelectric crystal, and a pair of independent resonators placed in close proximity on the piezoelectric crystal such that an efficacious portion of acoustic energy couples between the resonators. The first independent resonator serves as an input port through which an input signal is converted into mechanical energy within the sensor and the second independent resonator serves an output port through which a filtered replica of the input signal is detected as an electrical signal. Both a time delay and an attenuation at a given frequency between the input signal and the filtered replica may be measured as a sensor output. The sensor may be integrated into an assembly with a series feedback oscillator and a radio frequency amplifier to process the desired sensor output. In the preferred embodiment of the invention, a selective film is disposed upon the grounded metal layer of the sensor and the resonators are encapsulated to isolate them from the measuring environment. In an alternative embodiment of the invention, more than two resonators are used in order to increase the resolution of the sensor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to sensing chemical, biochemical and physical 
measurands using piezoelectric sensors. 
2. Description of the Related Art 
There are a number of sensor applications that demand exceptional 
physically, chemically and environmentally stable sensing elements. Two 
classes of sensors which are frequently proposed to this end are the 
optical fiber based sensors and the piezoelectric sensors, both of which 
derive their chemical and physical stability from solid state construction 
of inert glassy or crystalline solids. One advantage of the piezoelectric 
sensors is their wide range of potential sensing mechanisms coupled with 
the exceptional temperature stability of the most common implementation, 
namely quartz crystal technology. 
Various piezoelectric sensor geometries have been proposed and patented. 
The most widely considered of these is the quartz crystal microbalance, 
though other piezoelectric crystals, polymers and composites have also 
been used. Though the original patents in this field are expired, the 
technology is still not a mature technique for many sensing applications. 
U.S. Pat. No. 4,760,351, issued to Newell et al., teaches the use of arrays 
of these devices which consist of a parallel plate capacitor employing a 
piezoelectric material as the dielectric support. While this structure has 
exhibited good sensing characteristics in the vapor phase, sensor 
performance is substantially impaired when fluid phase operation is 
pursued. 
U.S. Pat. No. 5,374,521, issued to Kipling et al., discloses an alternative 
mode of operation of these sensors intended to overcome these 
difficulties; however, the method is not amenable to field deployment or 
mass production. 
The preferred methods of operating the crystal sensor are (1) using an 
impedance analyzer in a laboratory setting or (2) incorporating the device 
into an oscillating circuit. Under vapor phase operation, both techniques 
are suitable; however, liquid phase operation incurs many difficulties in 
instrumentation, which substantially impair the reliability and 
sensitivity of the sensor. 
U.S. Pat. No. 4,847,193, issued to Richards et al., discloses a signal 
amplification technique which partially overcomes this problem in selected 
assays. U.S. Pat. No. 5,179,028, issued to Vali et al. discloses an 
alternative structure which replaces the parallel plate capacitor with a 
tuning fork geometry. This geometry appears to offer other measurement 
methods but does not overcome the difficulties associated with liquid 
phase operation. 
U.S. Pat. No. 4,735,906, issued to Bastiaans, discloses a surface wave 
device which supports two separate interdigital transducers which serve to 
convert energy between electrical and acoustic signals. These transducers 
employ arrays of electrodes which are periodically spaced on the surface 
of the crystal and are alternately connected to the positive and negative 
terminals of the electrical input or output. 
U.S. Pat. No. 5,306,644, issued to Myerholtz et al. teaches an improvement 
on Bastiaans invention by employing a specific combination of materials 
and structural design to substantially reduce mechanical interactions with 
the fluid medium. Myerholtz et al. also discloses the expansion of a 
single sensor to an array. 
U.S. Pat. No. 5,478,756, issued to Gizeli et al. teaches an alternative 
improvement on the Bastiaans structure which employs a thick film of 
dielectric material to help confine the acoustic signal to the surface. 
All these approaches suffer from two significant limitations. The foremost 
of these is that the structures do not isolate the electrical connections 
from the sensing environment. One solution has been to employ an acoustic 
waveguide version of the Bastiaans structure. While this structure 
overcomes the most substantial limitations of the Bastiaans structure, it 
incurs additional spurious resonances which further complicate the sensor 
instrumentation. Finally, all of the Bastiaans-derived structures require 
micron scale lithography and teach towards complicated, surface-based 
devices. A simpler implementation is based upon the (quartz) crystal 
microbalance devices. The fundamental problem with these techniques, 
however, is that the sensing element is a simple reactive electrical 
component with a single electrical "port". 
There is not found in the prior art a sensing apparatus which has no 
additional spurious resonances, separate input and output terminals, and 
isolates the electrical connections to the circuit from the sensing 
environment. 
SUMMARY OF THE INVENTION 
It is an aspect of the invention to provide a sensing apparatus which has 
separate input and output terminals. 
It is another aspect of the invention to provide a sensing apparatus which 
isolates the electrical connections to the circuit from the sensing 
environment. 
The invention is an improvement on the (quartz) crystal resonator which 
employs the parallel plate configuration while isolating the non-grounded 
electrical connections from the sensing medium and providing separate 
input and output electrical ports. Its relationship to the existing 
resonator is analogous to the relationship of the transistor to a diode. 
Other aspects and advantages of the present invention will become apparent 
and obvious from a study of the following description and the accompanying 
drawings which are merely illustrative of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
The sensor is a novel piezoelectric apparatus that is useful for sensing 
chemical, biochemical and physical measurands. The sensor operates by 
measurement of the perturbations of the electrical parameters of a 
piezoelectric sensing element. The sensing element is capable of detecting 
both electrical (e.g. conductivity and permittivity) and mechanical (e.g. 
elasticity, viscosity and density) perturbations caused by a measurand. 
The sensing element consists of two or more acoustically coupled resonant 
structures on a single piezoelectric substrate or a substrate coated with 
a piezoelectric layer. The resulting structure, known as a monolithic 
filter, provides separate input and output electrical connections which 
are physically separated from the common ground electrode, allowing it to 
be incorporated into a large number of diverse measurement systems. The 
sensor is attractive for sensing a variety of physical, chemical and 
biochemical measurands, in particular for trace chemical analysis, 
including trace vapors, metals and biochemicals--either in solution or in 
gaseous mixtures--and physical measurands such as acceleration, pressure, 
etc. 
The invention combines the best aspects of (quartz) crystal resonator and 
other piezoelectric sensor techniques and is a fundamental improvement to 
the well known quartz crystal microbalance (QCM). While the invention has 
some of the physical traits of the well known QCM, it also overcomes many 
limitations of the QCM, especially with respect to fluid phase sensing and 
sensing in highly corrosive environments. 
FIG. 1A illustrates the structure of the traditional QCM device, as it 
would be employed for fluid-loaded or corrosive environment operation. The 
only relevant design parameters are the electrode shape, area and the 
plate thickness. The substrate material is almost always AT-cut quartz, 
which is chosen for its excellent temperature stability. As shown, the 
traditional crystal sensor consists of a piezoelectric crystal with metal 
electrodes on both surfaces. The region between the electrodes supports a 
mechanical standing wave (denoted by the parallel lines). Electrical 
connections are made to the grounded side through the case and to the 
positive side via a wire point contact. 
FIG. 1B is a schematic of the equivalent electrical circuit, which consists 
of a series resonant circuit in parallel with a capacitance. For most 
frequencies the device appears to be a capacitor, except in a narrow band 
of frequencies near the resonant frequency of the series branch of the 
circuit. 
The resonant frequency of the crystal is determined by the ratio of the 
mass (embodied in the equivalent circuit as the inductance) to the 
stiffness (embodied in the equivalent circuit as the capacitance) and the 
thickness of the plate. Added mass--which becomes bound to the face of the 
crystal--increases the inductive component and thus lowers the resonant 
frequency in a predictable and measurable fashion. It is possible to 
quantitatively detect nanogram per mm.sup.2 and lower levels of mass 
loading using this technology. The piezoelectric sensor is, itself, a 
nonspecific mass detector; however, excellent specificity is added by 
coating the crystal with a selective film, e.g., biochemical receptor 
film, such as monoclonal antibody, DNA oligonucleotide probes or other 
suitable bioreceptors or polymer or metallic films. Residual nonspecific 
binding may be compensated by employing a second, reference crystal which 
is coated with a different film having similar electrical (charge vs. pH) 
and mechanical properties. Typically, a second selective receptor is 
selected which is insignificant to the detection process at hand. Thus, 
specific tracts of bacterial DNA might be detected using a specific 
sequence on the sensor crystal while employing a nonsense sequence on the 
reference sensor. 
The most common and cost-effective method of monitoring the resonant 
frequency employs the crystal as the stabilizing element in an oscillator 
circuit. The circuit depends on the unique characteristics of the crystal 
at its resonant frequency. In signal processing and frequency control 
applications the preferred oscillator circuit places the crystal as a 
shunt feedback element (between the input and output terminals of an 
amplifier) and oscillates on the series resonance. This arrangement places 
both electrodes of the QCM at an active electrical potential and incurs 
deleterious effects from packaging methods such as that depicted in FIG. 
1. To eliminate these effects one of the electrodes must be connected to 
ground. While this is not inherently a problem, it severely limits the 
classes of oscillator circuits which may be employed to measure the 
resonant frequency. In fact, all of the well-known circuits which may be 
employed with the QCM having one electrode grounded operate as image 
impedance oscillators. These circuits employ a transistor or other active 
device in an unstable mode of operation and employ the crystal to control 
the instability. For example, an inductive component between a 
transistor's base and ground will induce instability. This class of 
circuits typically requires that the net electrical characteristics are 
inductive at the frequency of oscillation. This is difficult to ensure 
under fluid loading, in which case the resonant branch of the circuit is 
highly damped and the parallel capacitor dominates. Other 
instability-based oscillators employ a capacitive element between a 
transistor's emitter and ground. While the crystal can serve this role, it 
is capacitive at virtually all frequencies and is thus not a reliable 
method of stabilizing an oscillator. 
As previously stated, oscillator circuits which employ the QCM as a shunt 
feedback element have excellent stability characteristics. Typically, 
these circuits employ unconditionally stable amplifiers which provide 
adequate gain to overcome the resistive losses in the QCM. When the 
electrical phase shift around the loop is a multiple of 2.pi. radians, 
signals are successively amplified on each transit around the loop until 
the transistor saturates. Since noise signals are capable of starting this 
process, such a circuit invariably oscillates. Unfortunately, all of the 
circuits which employ this mode of operation require that two separate 
electrodes be electrically active with respect to ground. This has proven 
extremely problematic when using the packaging proposed in FIG. 1 or any 
suitable alternative. The fundamental constraint is that the QCM is a 
two-terminal device and one of the terminals must be placed in contact 
with the fluid. This fluid-loaded electrode must be grounded for reliable 
electrical operation. 
FIG. 2 illustrates the monolithic piezoelectric sensor (MPS) invention, 
which is based on the (quartz) multi-resonator monolithic filter 
technology. The MPS employs two driven electrodes referenced to a grounded 
metal layer (top). The grounded metal layer (typically gold) may be coated 
with covalently attached bioreceptors (e.g. antibody, enzyme or DNA), or 
with any other materials that selectively bind their analyte. The added 
mass changes the frequency response in a predictable and measurable 
fashion. This shift is typically monitored by embedding the sensor in a 
series feedback oscillator. The three-terminal MPS may be employed in 
series feedback oscillators while still allowing sensing to occur on a 
grounded, physically-isolated electrode. The invention comprises two or 
more independent resonators placed in close proximity on the same crystal, 
such that some acoustic energy couples between them. 
The net result is a sharp, well defined filter function, as shown in FIG. 
3. Electrical transmission and phase is plotted as a function of 
frequency. When placed in the feedback path of an amplifier, the resulting 
circuit can be induced to oscillate at a fixed frequency for which there 
is net gain around the loop and the phase shift is a multiple of 360 
degrees. Extremely stable oscillation occurs when the frequency is 
designed to be in the center of the resonance, in the area of high phase 
slope. Since the entire response shifts with mass loading, stable 
operation is maintained over all cases of sensor operation. 
The electrical signal applied to the input appears at the output with a 
finite attenuation and a well-defined phase shift. By placing the device 
in the feedback path of a radio-frequency (RF) amplifier, the stable 
resonance of the crystal controls the frequency of oscillation in the 
circuit. 
The only substantial effects of loading the crystal with a viscous fluid 
are to slightly increase the attenuation and to lower the oscillation 
frequency proportional to the viscoelastic properties of the liquid and 
the added mass due to accumulation of the target chemical in accordance 
with well-known mathematical models. It is known that during surface 
reactions, detectable levels of mass change occur on the face of the 
crystal. Therefore, by measuring the frequency change and the attenuation 
change of the oscillator, one can quantitate surface reactions and . . . 
using calibration data . . . determine solution titer. 
In the case of loading the MPS with conductive materials, such as 
conductive liquids or thin solid films, the coupling impedance, Zc of FIG. 
4, undergoes changes due to the changes of both the dielectric constant 
and the conductivity of a loading material. Several chemical, biochemical 
and physical processes change the electric properties of materials. 
Examples include sensing gas by thin metal oxide films or quantitating 
metal ions in water. As a result, the resonant frequency and the amplitude 
of the MPS vary accordingly allowing for determination of the acting 
measurands. 
A MPS sensor offers a very attractive apparatus for sensing a variety of 
physical measurands such as pressure, acceleration, electric field, 
temperature, etc. In this case, the measurand of interest directly changes 
the MPS substrate parameters such as elastic, dielectric or piezoelectric 
constants. As a result, the frequency and amplitude of the MPS changes 
accordingly. For example, the pressure or electric field applied to the 
MPS changes the elastic constants of the substrate via nonlinear 
elasto-acoustic and electro-acoustic effects. 
In its simplest form, the sensor comprises a piezoelectric plate with at 
least two resonators placed in proximity, as shown in FIG. 2. Each 
resonator comprises two electrodes--placed on opposite surfaces for 
in-line excitation, as shown, or on the same surface for parallel 
excitation (not shown). The electrodes consist of a conductor, such as 
gold, silver or aluminum. The selection of the electrode metal is 
determined by the environmental issues associated with the specific 
application and with the frequency of operation. Very high frequency 
devices tend to employ aluminum for its superior mechanical properties. 
Lower frequency devices typically prefer silver or gold for environmental 
stability (corrosion resistance). The design rules for electrode thickness 
are determined by energy trapping theory, as is the spacing between the 
adjacent resonators. A variety of electrode shapes are possible. 
Traditionally, rectangular electrodes are employed in analogous signal 
processing devices for mathematical simplicity; however, elliptical and 
circular electrodes are also known in these applications. Sensor elements 
are feasible with any possible electrode geometries, including elliptical, 
rectangular, toroidal, etc. 
In this configuration, one resonator serves as an input port through which 
electrical energy is converted to mechanical energy within the sensor. The 
second resonator serves as an output port, through which a filtered 
(delayed and attenuated) replica of the input signal is detected as an 
electrical signal. Both the delay time and the attenuation at a given 
frequency may be affected by the measurand and may be measured as the 
sensor output. 
The structure of FIG. 3 would be sensitive to physical (e.g. acceleration, 
pressure), electrical (e.g. conductivity, etc.) and mechanical (e.g. mass 
loading or viscoelastic loading) perturbations. Selective films may be 
employed on either surface of the sensor or on both surfaces 
simultaneously, allowing three configurations of the sensor. The sensor 
may be packaged so as to expose either or both surfaces to the test 
environment. Although any of these configurations is acceptable, the 
preferred approach is to place the film on the grounded surface and 
enclose the electrical connections (input and output electrodes) in 
isolation. 
In some cases, it would be preferable to eliminate electrical 
perturbations, for example, in a mass-based sensor. This may be 
accomplished by employing a continuous ground electrode, as shown in FIG. 
2, and encapsulating the active input and output electrodes. Since the 
electric field is shorted everywhere on the sensing surface, spurious 
changes in the conductivity of the test solution would not cause 
deleterious sensor drift. 
Finally, in some applications and/or instrumentation methods it may be 
desirable to employ a multi-pole filter consisting of more than two 
resonators in proximity. The primary advantage of this approach is to 
increase the phase slope versus frequency, allowing better resolution. The 
ability to resolve phase changes increases approximately linearly with the 
number of resonators. 
It is feasible to place different films over the different resonators and 
to employ spectroscopic measurements and neural network techniques to 
deconvolve the individual perturbations. 
While there have been described what are at present considered to be the 
preferred embodiments of this invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the invention and it is, therefore, aimed 
to cover all such changes and modifications as fall within the true spirit 
and scope of the invention.