Acoustic wave device using plate modes with surface-parallel displacement

Solid-state acoustic sensors for monitoring conditions at a surface immersed in a liquid and for monitoring concentrations of species in a liquid and for monitoring electrical properties of a liquid are formed by placing interdigital input and output transducers on a piezoelectric substrate and propagating acoustic plate modes therebetween. The deposition or removal of material on or from, respectively, a thin film in contact with the surface, or changes in the mechanical properties of a thin film in contact with the surface, or changes in the electrical characteristics of the solution, create perturbations in the velocity and attenuation of the acoustic plate modes as a function of these properties or changes in them.

BACKGROUND OF THE INVENTION 
Acoustic waves (AW) in piezoelectric structure have been used to measure 
liquids in contact with the piezoelectric structure. For example, U.S. 
Pat. No. 4,378,168 of H. Kuisma, Mar. 29, 1983, discloses a piezoelectric 
device having spaced input and output electrodes on a surface for 
generating a surface acoustic wave (SAW) that detects the presence of 
humidity between the electrodes as a function of signal attenuation. 
However, this device does not provide an indication of any properties of 
condensed liquid. 
The use of devices which use the Rayleigh wave, or SAW, to sense mass 
changes at solid/gas interfaces is known. However, the SAW is impractical 
for use in detection at solid/liquid interfaces because the Rayleigh wave 
they employ does not propagate efficiently at a solid-liquid interface. 
The Rayleigh wave has a surface-normal component of particle displacement 
which generates compressional waves in a liquid, leading to substantial 
attenuation of the wave. 
U.S. Pat. No. 4,691,714 of J. Wong et al., Sep. 8 1987, discloses the use 
of a bulk acoustic wave within a piezoelectric structure to measure 
viscosity of a liquid in contact with the structure. The viscosity of the 
liquid is a function of the amplitude of the transmitted bulk acoustic 
signal, and the temperature of the liquid (actually, the temperature of 
the sensing transducer) is a function of the phase of the SAW that is also 
transmitted in this device. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an acoustic wave sensor 
for monitoring and quantifying the concentration of species in a solution 
by chemically modifying the sensor surface with a material or substance 
which selectively adsorbs or absorbs the solution species. 
It is a further object of the present invention to provide an acoustic wave 
sensor for monitoring and quantifying the deposition and removal of 
material from a surface immersed in a liquid. 
It is another object of the present invention to provide an acoustic sensor 
for monitoring and quantifying the variations in solution electrical 
characteristics, including ionic conductivity and dielectric coefficient. 
Additional objects, advantages, and novel features of the invention will 
become apparent to those skilled in the art upon examination of the 
following description or may be learned by practice of the invention. The 
objects and advantages of the invention may be realized and attained by 
means of the instrumentalities and combinations particularly pointed out 
in the appended claims. 
To achieve the foregoing and other objects, and in accordance with the 
purpose of the present invention, as embodied and broadly described 
herein, the present invention may comprise a piezoelectric substrate 
having a pair of opposed surfaces, with spaced input and output 
transducers affixed to the substrate for propagating acoustic plate modes 
through said substrate. Means are provided for maintaining a liquid in 
contact over a predetermined portion of a surface, or a film on a surface, 
of said substrate, the liquid creating perturbations in the velocity of 
the acoustic plate modes passing along the surface. An rf signal is 
applied to the input transducer and received at the output transducer. 
Means are also provided for determining the velocity perturbations from 
the rf signals, the perturbations being an indication of the conditions 
being monitored.

DETAILED DESCRIPTION OF THE DRAWINGS 
The purpose of this invention is to sense properties of solution species 
which may be dissolved in a liquid, deposited onto a surface immersed in a 
liquid, or removed from a solid surface immersed in a liquid. The 
invention is practiced by adsorbing, absorbing, or plating the solution 
species onto a acoustic wave device surface, or by desorbing, dissolving, 
or corroding a species from the device surface into solution, or by 
monitoring the extent and nature of acoustoelectric interactions between 
the AW and the solution and species in the solution. 
The invention is fabricated on a piezoelectric single or polycrystalline 
substrate having a surface portion which may be coated with a material 
providing enhanced selectivity and/or sensitivity to a particular solution 
species. In the case of corrosion and electro deposition sensors, the 
immersed region is coated with a metal or other conductive electrode 
material to which electrical contact is made for the purpose of monitoring 
or applying a potential or current during the process of corrosion or 
electro deposition. Change in the amount of species on the device surface 
is detected through the perturbation of an acoustic plate mode propagating 
through the crystal and having in-plane displacements at the solid-liquid 
interface. 
The capability and sensitivity of such detection and monitoring devices for 
sensing in liquids is dependent upon the use of the appropriate acoustic 
mode. The optimum mode in the present invention consists of one, or a 
superposition of more than one, acoustic plate modes (APMs). Plate modes 
are so-called because the acoustic wave reflects back and forth between 
the two faces of the crystalline plate while propagating along the length 
of the crystal. 
On certain substrates, including particular cuts of quartz and lithium 
niobate, it is possible to excite acoustic plate modes by means of an 
interdigital transducer. In particular, APMs having displacement 
components at both crystal surfaces which lie only in the plane of these 
surfaces, and which therefore propagate efficiently at a solid-liquid 
interface, can be excited. The APMs propagate efficiently in the presence 
of liquids due to the absence of surface-normal components of particle 
displacement. 
Addition of mass to either of the surfaces of the APM sensor slows the 
velocity of the acoustic wave to less than that of the unperturbed 
substrate. Conversely, if mass is initially added to either of the device 
surfaces, e.g., in the form of a thin film, the subsequent removal of a 
fraction or all of said mass causes the wave velocity to increase, the 
velocity reaching its unperturbed value when all the added mass has been 
removed. In addition, changes in the mechanical properties, including the 
stiffness, elasticity, and shear modulus, of a thin film in intimate 
contact with the APM sensor surface, result in perturbations of the 
acoustic wave velocity. Such changes in wave velocity resulting from 
change in surface mass and/or surface mechanical properties are analogous 
to those experienced by conventional SAW sensors operating in contact with 
the gas phase. The difference in the case of this invention is that these 
mass and mechanical changes can be readily measured while the device is in 
direct contact with a liquid, a measurement not readily made with a 
Rayleigh wave device. 
Referring in detail to the drawings and with particular reference to FIGS. 
1 and 2, the solid-state acoustic sensor is shown as including a generally 
rectangular piezoelectric substrate 12 with parallel front and back 
surfaces 12FS and 12BS on which are positioned an interdigital input 
transducer array 14 and an interdigital output transducer array 16. The 
arrows 18 within the substrate 12 indicate some, but not all, of the paths 
which may be taken by the APMs transmitted from the input transducer 14 to 
the output transducer 16 when an appropriate electrical signal is applied 
to input transducer array 14, as discussed hereinafter. 
The front surface 12FS of the substrate 12 carries the input and output 
transducer arrays 14 and 16, which arrays are respectively positioned at 
opposite ends of the rectangular substrate 12. Each of the transducer 
arrays is formed of interdigitated elongated sets of fingers 14A-14B and 
16A-16B arranged perpendicular to the plane containing the arrows 18 
representative of the path of the wave; i.e., the transducers are normal 
to the direction of propagation of the APMs. 
Either the front surface 12FS (FIG. 1) or the back surface 12BS (FIG. 2) is 
interfaced with a liquid LQ (shown by shading) confined in a cell 
structure 20 which comprises a liquid container for providing a liquid 
interface over a defined portion of one of the substrate surfaces 12FS or 
12BS. When the liquid is interfaced with the front surface 12FS, the cell 
containing said liquid is located intermediate the input and output 
transducer arrays 14 and 16. No such restriction is placed on the location 
of the cell when the liquid is interfaced with the back surface 12BS of 
the substrate, the only requirement being that the liquid LQ contact some 
portion of the propagation path of the APMs. 
A thin film 5, illustrated in each of FIGS. 1 and 2, may be placed between 
liquid LQ and substrate 12 as discussed hereinafter. 
For either embodiment, cell 20 may either be a relatively low-walled 
structure, as shown, into which a sample of liquid LQ may be placed. 
Alternatively, cell 20 may represent one end of a tubular structure (not 
shown), the other end of which structure being connected to a container of 
the liquid LQ under test. 
The input and output transducer pair 14, 16 defines an acoustic delay line. 
While only one such delay line is illustrated on the substrate 12, two 
such transducer pairs may be provided with one pair being a reference 
standard compensating for changes in ambient parameters such as 
temperature, pressure, density and the like, and the other pair, as 
illustrated, being for sensing the characteristics being monitored. 
While the APM sensor is in contact with a liquid, a fractional change in 
the velocity of the APMs will result from changes in sensor surface 
conditions including, but not limited to, changes in the characteristics 
of the sensor surface or a thin film of material upon said sensor surface 
due to adsorption, absorption, deposition, removal, or desorption of 
matter; or changes in the mechanical properties of a thin film or of the 
sensor surface itself. 
FIG. 3 illustrates one technique for measuring these fractional velocity 
changes. A radio frequency signal source 22 drives the fingers 14A, B of 
the input transducer 14 through an impedance matching transformer 34 to 
propagate the APMs from the input transducer 14 to the output transducer 
16. The output transducer is terminated through another matching 
transformer 36 by a 50 ohm load 38. A vector voltmeter or phase-sensitive 
meter 40 equipped with two probes A, B monitors the phase difference 
between the acoustic wave incident upon the input transducer 14 and that 
received by the output transducer 16, providing a measure of the 
perturbations in wave velocity. Probes A, B are coupled to their 
respective signals through directional couplers 41, 43, respectively. 
Another measurement technique is shown in FIG. 4, where output transducer 
16 is connected to input transducer 14 through an amplifier 42 suitable 
for operation at radio frequencies. A directional coupler 30 directs a 
fraction of the power incident on the input transducer 14 to a frequency 
counter 48 which provides a measure of the perturbations in wave velocity. 
A specific example of the preferred embodiment of FIGS. 1 and 2 has been 
constructed. Substrate 12 was a polished single crystal of ST-cut quartz 
0.5 mm (0.020) inches thick having a rectangular configuration 7.6 mm wide 
by 11.4 mm long (0.3.times.0.45 inches). The interdigital transducers 14A, 
B and 16A, B each had fifty (50) finger pairs wherein the fingers are 8 um 
wide and 1.675 mm long gold-upon-chromium lines, approximately 200 nm 
thick, separated by 8 um spaces and fabricated on the front surface 12FS 
of the substrate 12 by standard lithographic techniques. 
The acoustic synchronous frequency (158 MHz) of the transducers 14, 16 is a 
function of the periodicity, d (32 um), of the interdigital patterns 
thereof, and the substrate material and thickness. This frequency is 
determined experimentally from the constructed device. Spacing between the 
input and output transducers 14 and 16 of each delay line on the substrate 
12 is 6.4 mm (200 wavelengths). 
The resulting sensor device was mounted in an 12 mm.times.25 mm 
(0.5.times.1.0) inch flatpack. Electrical connections to the device were 
made with gold wires bonded between the flatpack leads and bonding pads on 
the transducers 14, 16. The cell 20 for receiving the liquid LQ was made 
of polytetrafluoroethylene (Teflon) held to the surface 12FS using 
pressure from a metal fitting. 
The input transducer 14, with periodicity d on substrate 12 of thickness b, 
excites efficiently a family of acoustic shear plate modes whose frequency 
spectrum is approximated by: 
EQU f.sub.n =(v.sub.o /2.pi.)[(2.pi./d).sup.2 +(n.pi./b).sup.2 ].sup.1/2(1) 
in which v.sub.o is the velocity of the acoustic wave in a solid in the 
absence of bounding surfaces (5100 m/s for ST-quartz) and n is an integer 
(1, 2, 3, . . . ). When the number of finger pairs N comprising the 
transducers 14, 16 is chosen such that N&gt;4(b/d).sup.2, the bandwidth of 
the transducers will be sufficiently narrow that each acoustic plate mode 
can be excited and detected individually; otherwise, a superposition of 
more than one plate mode is excited and detected. 
The APMs can be used to measure the mass and mechanical properties of 
material present upon the sensor surface, or deposited from (or removed 
into) a liquid in direct contact with either sensor surface. The 
perturbation in APM wave velocity depends both on changes in surface mass 
and stiffness properties, with the fractional velocity perturbation given 
by: 
EQU .DELTA.v/v.sub.o =-c.sub.m .delta..sub.m +F(.beta.,u) (2) 
in which c.sub.m, the mass sensitivity factor of the device, is given 
approximately by v.sub.o.sup.2 /(c.sub.44 b), where v.sub.o is the 
unperturbed operating velocity and c.sub.44 is a stiffness parameter for 
the plate material; .delta..sub.m is the accumulated mass per unit area; 
and F is a function of the bulk modulus .beta. and shear modulus u of 
material on the surface. 
When the wave velocity in any layer accumulating on or present on the 
sensor surface is much less than the wave velocity in the sensor substrate 
12, this layer and any changes in it will have a negligible effect on the 
acoustic wave velocity via mechanical properties. Equation 2 then reduces 
to a simpler form: 
EQU .DELTA.v/v.sub.o =-c.sub.m .delta..sub.m (3) 
In this case, the fractional change in velocity, and thus oscillation 
frequency, is linearly proportional to changes in surface mass. A 
convenient and precise way to monitor wave velocity is to incorporate the 
APM device as the feedback element in an oscillator loop as shown in 
detail in FIG. 4. The oscillation frequency of such a loop tracks and is 
linearly proportional to the acoustic wave velocity, so that fractional 
changes in the latter can be measured with parts per billion accuracy. The 
fractional change in frequency is equal to the fractional change in 
velocity given in Equations 2 and 3, above, times the fraction of the APM 
propagation path which experiences a change in mass and/or mechanical 
properties. 
The signal represented by Equations 2 and 3 adds to the (constant) shift 
arising from the entrainment of liquid through viscous coupling. The APM 
sensitivity is greatest when b, the plate thickness, is minimized. 
Referring now to FIGS. 1 and 2, the sensor 10 of the present invention is 
shown as it is adapted for sensing electrode reactions such as plating, 
corrosion, and the like, wherein the fluid LQ in the cell 20 is an 
electrolyte and the surface 12FS, BS of the portion of substrate 12 
subtended by cell 20 is covered with a thin film 5 constituting an 
electrode or reacting surface. In a similar manner, monitoring of 
so-called electroless plating, in which a chemical reaction within the 
electrolyte solution results in the deposition of metal on a surface 
contacting the solution, may be accomplished without the need for a metal 
thin film on the sensor surface 12FS, BS provided the sensor surface is 
first sensitized with the chemicals commonly used to sensitize glass or 
plastic surfaces for such electroless metal deposition. 
The thickness of any material(s), i.e., the thin film 5, deposited on the 
substrate should be no more than a few percent of the 32 um acoustic 
wavelength, thereby insuring that changes in wave velocity are linearly 
related to mass changes. Thus, the thickness of the electrode, together 
with any electro deposited materials, may range from less than one 
Angstrom to approximately 1 um. Pd, Au, Al, and Cu have thus far been used 
as electrode materials. 
To demonstrate the measurement of mass accumulated from solution, silver 
was electro deposited onto a palladium electrode (vacuum evaporated onto 
the quartz substrate). Since the deposition rate is controlled by the 
electrode current (galvanostatic control), direct calibration of the APM 
mass sensitivity was obtained by comparing the frequency shift with the 
charge passed for a plating process of known current efficiency. For the 
silver plating experiment, which has current efficiency near unity, 
electrolyte containing 0.3M [Ag(CN).sub.2 ].sup.- and 0.3M free CN.sup.- 
in a basic solution was added to the electrochemical cell 20; a Ag wire 
served as counter electrode. Current density was -0.3 mA/cm.sup.2. 
FIG. 5 shows the fractional frequency change as a function of the charge 
passed for this embodiment. The APM monitor reveals an induction period 
during which no significant change in surface mass occurs. This period may 
be due to reductive dissolution of surface impurities and/or Pd hydride 
formation. After the induction period, the frequency changes at a rate of 
1.0 ppm/uC. 
To demonstrate the monitoring of an etching process, the dissolution of a 
4200 Angstrom thick aluminum film in a 0.3% NaOH solution was followed as 
a function of time. A total frequency shift of 1280 ppm occurred over a 
period of approximately 4 min. With the exception of a few seconds at the 
beginning of the dissolution process, the etch rate was constant at 17 
Angstroms/s until the Al had been completely removed. 
Referring again to FIGS. 1 and 2, the sensor element 10 of the present 
invention may also be used to sense dissolved solution species by 
chemically derivatizing the quartz surface. For example, if the liquid is 
a solution containing dissolved species such as Cu.sup.2+ and the thin 
film is a surface-immobilized reagent such as 
N-2-aminoethyl-3-aminopropyltrimethoxysilane, suitable to ligate said 
solution species, the perturbation in velocity of the acoustic plate modes 
will be a function of variations in concentration and identity of the 
solution species, the variations being reflected in the mass bound by the 
thin film on the device surface. 
When a plate mode propagates in the piezoelectric quartz waveguide, the 
resulting mechanical deformation leads to the generation of evanescent rf 
electric fields which extend into the adjacent liquid environment. In a 
non-conductive liquid, the electric field decays exponentially with 
distance into the liquid, with a decay length of .lambda./2.pi. (.lambda. 
is the acoustic wavelength, 32 um). The mode velocity and attenuation are 
influenced by the interaction of this evanescent electric field with ions 
and dipoles in solution. 
The velocity changes and attenuation .alpha. (per wavenumber k) arising 
from the interaction of the evanescent electric field with ions in 
solution are: 
##EQU1## 
in which K.sup.2 is the electromechanical coupling factor, a measure of 
the piezoelectric strength of the substrate; .epsilon. is the dielectric 
coefficient of the substrate s, liquid l, and free space o; .omega. is the 
angular frequency of oscillation, and .sigma. is the ionic conductivity of 
the solution. 
Data have been obtained on the acoustoelectric interaction between plate 
modes and ions in solution. Velocity and attenuation were monitored while 
varying the ionic strength of a solution in contact with the device 
surface. While the attenuation change was too small to measure accurately, 
velocity decreased with increasing ionic strength, with the effect 
eventually saturating. Data are shown in FIG. 6 along with the result 
predicted by Equation 4a. The range of conductivities over which this 
effect is measurable, 0.01 to 0.3-.OMEGA.-cm, should prove useful for 
monitoring ionic strength. It is apparent from FIG. 6 that the dielectric 
properties of the liquid will also influence the propagation velocity of 
the plate mode. 
As can be seen from the foregoing specification and drawings, the present 
invention provides a new and novel solid-state acoustic sensor device for 
monitoring a variety of conditions at a liquid/surface interface. It 
should be noted that the cell 20 and reactive thin film 5 can be placed on 
either side of the sensor substrate 12, and that utilization of the back 
surface of the sensor 12BS allows protection of the transducer arrays 14 
and 16 from corrosive environments, if desired. Thus, the versatility of 
the sensor devices is further enhanced. 
The present invention having been thus described, it should be apparent 
that modifications could be made to the various components of the system, 
as would occur to one of ordinary skill in the art without departing from 
the spirit and scope of the present invention.