Characterization of Epoxy Resin (SU-8) Film Using Thickness-Shear Mode (TSM) Resonator under Various Conditions

Characterization of an epoxy resin film, commonly known as SU-8, is presented using thickness shear mode (TSM) quartz resonator. The impedance-admittance characteristics of the equivalent circuit models of the unperturbed and coated resonators are analyzed to extract the storage modulus and loss modulus (G\u27 and G\u27\u27). Those parameters are needed to establish SU-8 film as an effective wave-guiding layer in the implementation of guided shear-horizontal surface acoustic wave (SH-SAW) sensor platforms. Both cured and uncured polymer films are studied at the fundamental and third harmonic frequencies of the TSM resonators. The storage modulus (G\u27) and loss modulus (G\u27\u27) of the SU-8 film approach constant values of 1.66 × 1010 dyne/cm2 and 6.0 × 108 dyne/cm2, respectively, for relatively thicker films (\u3e20 μm) at a relatively low frequency of 9 MHz. The most accurate values for the extracted shear moduli G (G = G\u27 + jG\u27\u27) are obtained at high thickness where the viscoelastic contribution to the TSM response is substantial. The effect of temperature on the storage and loss moduli is determined for the range of −75 to 40 °C. It is found that the polymer approaches a totally glassy state below −60 °C. Exposure to water appears to follow Fickian diffusion behavior at short times and this exposure also results in changes to both G\u27 and G\u27\u27. However, stability is rapidly reached with exposure to water, indicating relatively lower water absorption, consistent with the extracted diffusion coefficient

Analysis of Liquid-Phase Chemical Detection Using Guided Shear Horizontal-Surface Acoustic Wave Sensors

Direct chemical sensing in liquid environments using polymer-guided shear horizontal surface acoustic wave sensor platforms on 36° rotated Y-cut LiTaO3 is investigated. Design considerations for optimizing these devices for liquid-phase detection are systematically explored. Two different sensor geometries are experimentally and theoretically analyzed. Dual delay line devices are used with a reference line coated with poly (methyl methacrylate) (PMMA) and a sensing line coated with a chemically sensitive polymer, which acts as both a guiding layer and a sensing layer or with a PMMA waveguide and a chemically sensitive polymer. Results show the three-layer model provides higher sensitivity than the four-layer model. Contributions from mass loading and coating viscoelasticity changes to the sensor response are evaluated, taking into account the added mass, swelling, and plasticization. Chemically sensitive polymers are investigated in the detection of low concentrations (1-60 ppm) of toluene, ethylbenzene, and xylenes in water. A low-ppb level detection limit is estimated from the present experimental measurements. Sensor properties are investigated by varying the sensor geometries, coating thickness combinations, coating properties, and curing temperature for operation in liquid environments. Partition coefficients for polymer-aqueous analyte pairs are used to explain the observed trend in sensitivity for the polymers PMMA, poly(isobutylene), poly(epichlorohydrin), and poly(ethyl acrylate) used in this work

Love-Wave Biosensors Using Cross-Linked Polymer Waveguides on LiTaO{sub 3} Substrates

The design and performance of Love-wave sensors using cross-linked poly-(methyl methacrylate) waveguides of thickness of 0.3--3.2 {micro}m on LiTaO{sub 3} substrates are described. It is found that this layer-substrate combination provides sufficient waveguidance, and electrical isolation of the IDTs from the liquid environment to achieve low acoustic loss and distortion. In bio-sensing experiments, mass sensitivity up to 1,420 Hz/(ng/mm{sup 2}) is demonstrated

SH-SAW Sensor Platform for Chemical Detection in Aqueous Solutions

Chemically sensitive polymers coated on delay lines utilizing shear horizontal surface acoustic wave (SH-SAW) sensors are investigated for the detection of organic analytes in liquid environments. The SH-SAW sensor platform was designed and fabricated on 36{degree} rotated Y-cut LiTaO{sub 3}. By depositing a SiO{sub 2} dielectric layer over the entire device prior to applying the polymer film, partial electrical passivation of the interdigital transducers (IDT) is obtained while increasing the mass sensitivity of the device. Changes in the mechanical properties of the chemically sensitive polymer materials were clearly detectable through a frequency shift at least one order of magnitude larger than that of a coated-quartz crystal resonator (QCR) in a similar experiment

Equivalent-Circuit Model for the TSM Resonator with a Viscoelastic Layer

This paper describes a new equivalent-circuit model for the thickness shear mode resonator with a surface viscoelastic layer operating near film resonance. The electrical impedance of the film is represented by a simple three-element parallel circuit containing a resistor, a capacitor, and an inductor. These elements describe the film's viscous power dissipation, elastic energy storage, and kinetic energy storage, respectively. Resonator response comparisons between this lumped-element model and the general transmission-line model show good agreement over a range of film phase conditions and not just near film resonance

Modeling the responses of TSM resonators under various loading conditions

The authors developed a general model that describes the electrical responses of thickness shear mode resonators subject to a variety of surface conditions. The model incorporates a physically diverse set of single component loadings, including rigid solids, viscoelastic media, and fluids (Newtonian or Maxwellian). The model allows any number of these components to be combined in any configuration. Such multiple loadings are representative of a variety of physical situations encountered in electrochemical and other liquid phase applications, as well as gas phase applications. In the general case, the response of the composite load is not a linear combination of the individual component responses. The authors discuss application of the model in a qualitative diagnostic fashion to gain insight into the nature of the interfacial structure, and in a quantitative fashion to extract appropriate physical parameters such as liquid viscosity and density, and polymer shear moduli