A.T. Tran

The extraordinary role of the AlN interlayer in growth of AlN sputtered on Ti electrodes

The structure of AlN layers grown on Ti with and without an AlN interlayer between the Si substrate and the Ti layer is investigated. The AlN grains take over the orientation of the Ti columnar grains in both cases. Surprisingly, the Ti grains do not take over completely the orientations of the AlN grains of the interlayer, and show the same columnar grain structure as the sample without interlayer. Hence, the structure of the AlN top layer is independent of the presence of an AlN interlayer below the Ti layer and is mainly determined by the Ti layer microstructure.

Encapsulated Aluminum Nitride SAW devices for liquid sensing applications

Very low-stress and good crystallinity Aluminum Nitride (AlN) thin films deposited on (100) Silicon substrates are optimized for surface acoustic wave (SAW) devices. We developed a simplified process to fabricate AlN SAW devices suitable for fast screening of liquid substances as needed for health and environmental monitoring. By encapsulating the interdigital transducer (IDT) with a very thin SiN layer, a sufficiently large sensing area is available between the electrodes. This allows direct and fast loading of the liquid drops on the device surface as well as cleaning, for repeated use. Moreover, the amount of layers on top of the piezoelectric AlN is minimized to avoid reduction of the signal. The fabricated devices are tested by using different buffer solutions typical in biological research (NaCl and phosphate buffered saline) and the possibility to operate these SAW devices for liquid sensors is demonstrated.

Sputtered molybdenum as conductive material for high-temperature microhotplates

This paper presents a fabrication process for high-temperature MEMS microhotplates that uses sputtered molybdenum as a conductive material. Molybdenum has a high melting point (2693°C bulk) and is simpler to deposit and pattern in larger series than platinum. Molybdenum is sensitive to oxidation above 300°C, so during fabrication it is protected by PECVD silicon oxide and then covered by LPCVD SiN. The electrical resistivity is linear with the temperature up to 700°C at least. Molybdenum microhotplate has a higher maximum operating temperature than platinum which is demonstrated by the observation of the boiling of barium carbonate (BaCO3) microcrystals at 1360°C. Annealing at 1100°C is effective in extending the operating range. The molybdenum microhotplate performs far better than platinum also in terms of long-term resistance drift.

SAW device for liquid vaporization rate and remaining molecule sensing

This paper presents an Aluminum Nitride (AlN) surface acoustic wave (SAW) device for liquid detection, based on remaining liquid molecules and vaporization rate. The sensing mechanism of the SAW device uses linear attenuation and frequency shift during the vaporization process of the liquid. As the device is meant to operate with liquids, the piezoelectric film and metal interconnects are protected by a silicon oxide layer. A change in insertion loss is measured when liquid drops (0.5-2 μl) of demi-water (DW), isopropyl alcohol (IPA) and ethanol (ETH) are applied on the surface of the SAW device. The selected liquids have different equilibrium vapor pressures and boiling points. The vaporization process of DW is slow enough for the network analyzer to measure linear changes in the insertion loss parameter. For IPA and ETH, there is a frequency shift caused by the appearance of remaining molecules on the surface after they vaporize. The center frequency is shifted by 0.25 MHz for ETH and 0.198 MHz for IPA.

Effect of the interruption of the propagation path on the response of surface acoustic wave transducers

This paper presents the effect of an arbitrary interruption of the propagation path in Surface Acoustic Wave (SAW) microdevices on the intensity of the scattered surface waves. Using finite element modeling, simulations have been carried out to validate a new equivalent circuit based on the conventional Mason and Smith model. In addition, experimental results obtained with 30, 50 and 100 μm diameter microholes are reported. The comparison of theory, simulation and experiment proves that it is possible to fabricate an interruption like deep microcavities or microholes in the propagation path which results in an acceptable signal magnitude attenuation, but without shift in the operating frequency.