Resumen de Thin film electroacoustic resonators for physical and chemical sensing
The fields of application of sensing devices are constantly expanding. From industrial processes monitoring to food quality control, sensors play a key role as information collectors, essential for human safeness, costs reduction and efficiency improvement. Their impact is even more pronounced in fields like the healthcare one, where biosensors have become crucial for prognosis and early diagnosis. As a result, high performance, low cost and reliable sensors are more and more demanded. In this context thin film electroacoustic sensors offer high resolution, high sensitivity, simplicity, low size, low manufacturing costs and possibility for array integration. Among this class of devices, thin film bulk acoustic wave resonators (FBARs), and more recently, Lamb wave resonators (LWRs) have attracted considerable attention. In both cases, although their sensing performance in different environments, namely gaseous and liquid, have been already studied, they have still unexplored features or improvement possibilities. This thesis aims at contributing to the field of thin film electroacoustic sensors by expanding the physical and chemical sensing features of AlN-based FBARs, particularly solidly mounted resonators (SMRs), and S0 mode LWR (S0-LWRs).
On one hand, Lamb wave devices exploiting the A0 mode, generally in delay line structures, have been widely studied in terms of their mass (gravimetric), but more deeply, of their in-liquid sensing mechanisms. However, the S0 mode is a potential candidate for in-liquid operation owing to its strong extensional component, which prevents radiation to the liquid. AlN-based S0-LWRs are relatively new devices and, although their gravimetric performance has been already studied, their sensing mechanisms in liquid environments have not been fully explored yet. Compared to FBARs that require tilted grains for their operation in liquids, S0-LWRs use c-oriented AlN, a deposition process already industrialized. This place them as the most promising candidates for commercialization. In this thesis the in-liquid sensing mechanisms of S0-LWRs are simulated through finite element analysis, and experimentally verified. Results show that S0-LWRs, in contrast to pure shear mode devices, are slightly more sensitive to the liquid density than to its viscosity. This arises from their different coupling mechanism to the liquid owing to the elliptical polarization, which, besides a strong extensional component, has a small vertical component. Moreover, if their membrane thickness is reduced their sensitivity is improved. Additionally, they are also sensitive to the dielectric permittivity of the liquid. However, a trade-off between technological feasibility and device performance should be always considered. A comparison between S0-LWRs and shear mode FBARs shows that S0-LWRs are more sensitive to the liquid mechanical properties when both devices work at the same resonant frequency (900 MHz), but also when FBARs work at higher frequencies (2 GHz).
On the other hand, FBARs, owing to their maturity, have been thoroughly investigated in both gaseous and liquid environments. However, much efforts have been put on the latter leaving their gas sensing potential with unexplored possibilities. In this thesis the integration of carbon nanotube (CNT) forest as sensing layers on AlN-based SMRs is proposed for boosting their gravimetric sensing performance. Vertically-aligned CNTs offer high surface area and a wide chemical affinity, suitable for sensor selectivity improvement. For the integration of CNT forests, two problems are solved in the thesis: SMRs holding high temperatures (CNT growth requires temperatures higher than 600ºC), and growth of CNTs on the metallic top electrodes of SMRs. The integrity of SMRs at high temperatures is guaranteed by a design including low stressed materials with high interlayer adherence. The successful growth of high quality CNT forests on metals like Mo or Ir is achieved by combining an Al under-layer with Fe catalyst, and using a temperature for nanoparticles formation lower than the one used for CNT growth. A proof of concept on the gravimetric sensing viability of CNT-based SMRs is demonstrated by detecting ethanol vapors.
