18-12-2012, 01:57 PM
Surface Acoustic Wave Probe for Chemical Analysis Introduction and Instrument Description
[attachment=44175]
Surface acoustic waves are easily propagated along a quartz
or lithium niobate surface. Interdigitized finger transducers
serve as transmitters of 30-60 MHz SAW waves toward an
equivalent detector. Surface molecules predictably affect the
wave propagation. Circuitry has been built to measure such
interactions by changes in amplitude or phase-angle shift of
the SAW, or the alteration of the resonance frequency when
the device is part of a "tank" circuit. Amplitude response is
proportional to the pressure of gaseous molecules in the
environment. For a given gas, the response factor is proportional
to (molecular weight)"*. Large linear frequency shifts
were noted as ambient pressures changed. Amplitude
measurements on quartz have the best S/N ratio (566).
Limited data suggest that LiNbO:, devices have considerably
better S/N ratios.
CONSTRUCTION OF APPARATUS
ST-Quartz Surface Acoustic Wave Device Fabrication.
The ST-quartz surface wave device used in most of the
studies conducted was not commercially available. The
substrate material was obtained from the Valpey Fisher
Corporation. Each ST cut, X propagating quartz substrate
was one-half inch wide, two inches long, and 35 mils thick.
One side was "SAW polished" (a surface roughness of less than
250 A). After cleaning, a 2300 f 50 A thick film of aluminum
was rf sputtered onto the smooth surface in preparation for
the fabrication of the interdigital transducers. Positive
photoresist was applied to the aluminum film. A photolith
mask with the 1:l scale transducer images on it was then
placed in contact with the substrate and illuminated. The
photoresist was developed, then baked, followed by etching.
The photolith mask with the transducer images on it was
also custom designed. Each transducer consisted of eight pairs
of electrodes of 1-mil width spaced on 2-mil centers. The
aperture of the fingers was 106 mils and the center to center
spacing of the two transducers was 1.84 inches.
Temperature, Control System. The thermomechanical
analysis of polymers required that accurate control of the
temperature of the SAW device be maintained. The use of
a simple on-off controller in this application was unacceptable.
A commercially available zero voltage firing proportional
temperature controller was used in this system (RFL Industries
model 72-115). Figure 10 illustrates the temperature
control system.
The
stainless steel detector block which accommodated the SAW
device and the previously described temperature control
equipment was incorporated into a system to perform thermal
analysis of the SAW devices themselves and films in contact
with them. This arrangement, shown in Figure 11, permitted
measurements to be made over the temperature range of 0
to 200 "C. The equipment used to control the pressure around
a SAW device is shown in Figure 12.
Data Acquisition Software. All of the SAW device
experiments used the LSI-11 microcomputer in some capacity.
The computer permitted very rapid data acquisition and
control, buffering of experimental data,
PRELIMINARY EVALUATION OF SYSTEM PERFORMANCE
Pressure vs. Amplitude Response Profiles. A quartz
SAW device was connected to the amplitude measurement
system and pressure test apparatus described. The vacuum
chamber was evacuated, purged, evacuated, and the test gas
was allowed to bleed in at a slow rate while the relative
amplitude was being monitored by the LSI-11. Helium,
nitrogen, air, carbon dioxide, nitrous oxide, and Freon-13 were
used as test gases. Multiple runs with each gas were performed.
The results obtained for helium, air, and Freon-13
at room temperature are illustrated in Figures 13a, b, c. A
plot of amplitude response (expressed in millivolts per 100
Torr) vs. molecular weight is shown in Figure 14
[attachment=44175]
Surface acoustic waves are easily propagated along a quartz
or lithium niobate surface. Interdigitized finger transducers
serve as transmitters of 30-60 MHz SAW waves toward an
equivalent detector. Surface molecules predictably affect the
wave propagation. Circuitry has been built to measure such
interactions by changes in amplitude or phase-angle shift of
the SAW, or the alteration of the resonance frequency when
the device is part of a "tank" circuit. Amplitude response is
proportional to the pressure of gaseous molecules in the
environment. For a given gas, the response factor is proportional
to (molecular weight)"*. Large linear frequency shifts
were noted as ambient pressures changed. Amplitude
measurements on quartz have the best S/N ratio (566).
Limited data suggest that LiNbO:, devices have considerably
better S/N ratios.
CONSTRUCTION OF APPARATUS
ST-Quartz Surface Acoustic Wave Device Fabrication.
The ST-quartz surface wave device used in most of the
studies conducted was not commercially available. The
substrate material was obtained from the Valpey Fisher
Corporation. Each ST cut, X propagating quartz substrate
was one-half inch wide, two inches long, and 35 mils thick.
One side was "SAW polished" (a surface roughness of less than
250 A). After cleaning, a 2300 f 50 A thick film of aluminum
was rf sputtered onto the smooth surface in preparation for
the fabrication of the interdigital transducers. Positive
photoresist was applied to the aluminum film. A photolith
mask with the 1:l scale transducer images on it was then
placed in contact with the substrate and illuminated. The
photoresist was developed, then baked, followed by etching.
The photolith mask with the transducer images on it was
also custom designed. Each transducer consisted of eight pairs
of electrodes of 1-mil width spaced on 2-mil centers. The
aperture of the fingers was 106 mils and the center to center
spacing of the two transducers was 1.84 inches.
Temperature, Control System. The thermomechanical
analysis of polymers required that accurate control of the
temperature of the SAW device be maintained. The use of
a simple on-off controller in this application was unacceptable.
A commercially available zero voltage firing proportional
temperature controller was used in this system (RFL Industries
model 72-115). Figure 10 illustrates the temperature
control system.
The
stainless steel detector block which accommodated the SAW
device and the previously described temperature control
equipment was incorporated into a system to perform thermal
analysis of the SAW devices themselves and films in contact
with them. This arrangement, shown in Figure 11, permitted
measurements to be made over the temperature range of 0
to 200 "C. The equipment used to control the pressure around
a SAW device is shown in Figure 12.
Data Acquisition Software. All of the SAW device
experiments used the LSI-11 microcomputer in some capacity.
The computer permitted very rapid data acquisition and
control, buffering of experimental data,
PRELIMINARY EVALUATION OF SYSTEM PERFORMANCE
Pressure vs. Amplitude Response Profiles. A quartz
SAW device was connected to the amplitude measurement
system and pressure test apparatus described. The vacuum
chamber was evacuated, purged, evacuated, and the test gas
was allowed to bleed in at a slow rate while the relative
amplitude was being monitored by the LSI-11. Helium,
nitrogen, air, carbon dioxide, nitrous oxide, and Freon-13 were
used as test gases. Multiple runs with each gas were performed.
The results obtained for helium, air, and Freon-13
at room temperature are illustrated in Figures 13a, b, c. A
plot of amplitude response (expressed in millivolts per 100
Torr) vs. molecular weight is shown in Figure 14