Development of a surface acoustic wave sensor for in situ detection of molecules

Surface acoustic wave sensors are highly sensitive micro acoustic devices which can be used as microactuators or detectors. In this work a mass detector based on surface acoustic waves has been developed which is suitable for the detection of molecules. The detector is based on a kind of delay line and mainly measures smallest changes of its resonance frequency, which is caused by the mass of molecules adsorbed on the delay line. Using the conventional piezoelectric material LiNbO$_{3}$ (Y cut, Z propagation direction) a resolution of the optimized mass detector has been achieved which is equivalent to a thickness resolution of a 3-aminopropyltriethoxysilan (APTES) layer of 0.01 nm. This has been achieved by optimizing the sample holder, sample design and sample mounting and by investigating and comparing their electronic properties via open and short tests. Furthermore our standard lift-off process for the electrode fabrication has been improved by an additional plasma ashing which led to a removal of residual contamination (most likely PMMA) underneath the electrodes and improved the mechanical adhesion.In order to further enhance the mass detectors’ resolution, the mass load sensitivity of epitaxial grown thin film K$_{0.7}$Na$_{0.3}$NbO$_{3}$ (Z propagation direction) on SmScO3 (110 cut) has been investigatedand compared to the conventional LiNbO$_{3}$ bulk material. At the same operating frequency both materials’ sensitivities seem to be identical. At low mass loads a linear frequency dependent regime has been observed with the sensitivity of c$_{m}$ = 0.11 m$^{2}$/(MHz kg). This is in agreement with the literature for LiNbO$_{3}$. At higher mass loads and/or frequencies a deviation of the linearity is observed which leads to a significantly increased sensitivity (factor 9). This regime might not only be of interest due to its higher sensitivity, it could also offer the possibility to use the sensor in liquids by transforming the Rayleigh-type sensor into a Love-type surface acoustic wave (SAW) via adding a wave guiding layer to generate so called Love waves. In addition, the SAW intensity distribution of the various harmonics showed that thin film KNNO seems to be applicable at higher frequencies which would lead to a further improvement in sensitivity. An attempt has been made to in situ monitor our molecular deposition and removal process of APTES with the developed SAW mass detector. The change of the detectors resonance frequency can monitor both processes. After the deposition the layer thickness of APTES has been determined to 0.35 nm assuming a molecular density equivalent to the liquid state. The frequency recording of the detector shows additional features when opening and closing the molecular source, which could provide further insights into the underlying physics of the deposition process itself. In conclusion, the optimized SAW mass detector is suitable for molecular detection, and its thickness resolution of 0.01 nm (with respect to the liquid state of APTES) could most likely be improved by thin film KNNO and/or by adding an wave guiding layer on the detector, which could also make it suitable for detection in liquids