Abstract
Temperature induced frequency shifts may compromise the sensor response of polymer coated acoustic wave gas-phase sensors operating in environments of variable temperature. To correct the sensor data with the temperature response of the sensor the latter must be known. This study presents and discusses temperature frequency characteristics (TFCs) of solid hexamethyldisiloxane (HMDSO) polymer coated sensor resonators using the Rayleigh surface acoustic wave (RSAW) mode on ST-cut quartz. Using a RF-plasma polymerization process, RSAW sensor resonators optimized for maximum gas sensitivity have been coated with chemosensitive HMDSO films at 4 different thicknesses: 50, 100, 150 and 250 nm. Their TFCs have been measured over a (−100 to +110) °C temperature range and compared to the TFC of an uncoated device. An exponential 2,500 ppm downshift of the resonant frequency and a 40 K downshift of the sensor’s turn-over temperature (TOT) are observed when the HMDSO thickness increases from 0 to 250 nm. A partial temperature compensation effect caused by the film is also observed. A third order polynomial fit provides excellent agreement with the experimental TFC curve. The frequency downshift due to mass loading by the film, the TOT and the temperature coefficients are unambiguously related to each other.
Highlights
The Rayleigh type surface acoustic wave (RSAW) mode has enjoyed considerable interest among the sensor community over the last three decades because of several unique features that are difficult or impossible to achieve with other technologies [1,2]
These data plots show that the stiff polymer film does shift down the device (TOT) and changes the slopes of the parabolic temperature frequency characteristics (TFCs) curve
The turn-over temperature (TOT) versus polymer thickness has a very similar exponential behavior as the resonance frequency shift that occurs as a result of increased mass loading on the RSAW device surface caused by the HMDSO film
Summary
The Rayleigh type surface acoustic wave (RSAW) mode has enjoyed considerable interest among the sensor community over the last three decades because of several unique features that are difficult or impossible to achieve with other technologies [1,2]. A two-port RSAW resonator coated with a chemosensitive polymer film makes an excellent high-resolution gas-phase sensor featuring fast response time, superb overall stability, high sensitivity and dynamic range and low sensor noise [3,4,5]. Even small fabrication tolerances in the metallization parameters from device to device on the same wafer or variations of the polymer thickness may result in turn-over temperature (TOT) differences of the delay lines used [14,15,16] This will result in significant measurement errors especially at the edges of the operating temperature range where small temperature variations result in large frequency shifts. Second and third order polynomial coefficients for precise interpolation of the frequency shift versus temperature dependence are extracted from experimental data on the tested devices and can be used successfully in the temperature compensation algorithm
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