Atomic layer deposition (ALD) of aluminum nitride (AlN) using in situ atomic layer annealing (ALA) is studied for microelectromechanical systems (MEMS). Effective piezoelectric in-plane actuation and sensing requires deposition of high crystal quality and (0002) oriented AlN on vertical sidewalls of MEMS structures. Previous studies have shown that the crystal quality of ALD AlN can be significantly improved using ALA but have not studied the conformal coverage or crystal quality on metal electrodes, which are required for piezoelectric MEMS devices. In this study, AlN thin films are deposited on Si, Al, Pt, and on vertical sidewalls etched into Si. The AlN microstructure and properties are studied using x-ray diffraction methods, transmission electron microscopy, and Fourier transform infrared spectroscopy. The conformal coverage is evaluated by measuring the film thickness on the vertical sidewalls. The effects of postdeposition annealing are studied as well. This study aims to enable effective piezoelectric actuation and sensing for MEMS sensors. The conformal coverage of the ALA ALD process is excellent and AlN has the best crystal quality and degree of orientation when deposited on Al. The as-deposited films contain oxygen impurities, which might be detrimental to the piezoelectric properties of AlN. Annealing at high temperatures reduced the number of impurities but did not improve the crystal quality.
Scandium-alloyed aluminum nitride (AlScN) is a potential material for micro-electromechanical systems because of its unique advantages, such as strong piezoelectric effect and high thermal stability. However, issues related to its stability and interaction with other materials in multilayer systems require investigation. The formation of new phases at the interface between piezomaterial and electrode material can lead to the device failure. In this study, multilayer structures Si substrate/AlN/Ti-Mo/Al 0.8 Sc 0.2 N/top electrode (TE) were studied after annealing at a wide range of temperatures and durations. Four different TE materials (i.e. Al, AlSi (1%), Mo/Al, and Mo) were examined to determine the most reliable electrode material for the structure. The phase stability, interfacial quality, and piezoelectric response of the multilayer systems after thermal annealing were investigated. The structure with Mo TE layer was stable after annealing at 800 • C for 300 h and at 1000 • C for 100 h. None of the structures formed any new phases at the interface between the electrode layer and AlScN. The transverse piezoelectric coefficient (e 31,f) was determined for Al 0.8 Sc 0.2 N before and after annealing. The absolute value of the e 31,f was −1.39 C/m 2 for as-deposited structure and −1.67 C/m 2 for the same structure annealed for 300 h at 800 • C.
Young's modulus of tapered mixed composition (zinc-blende with a high density of twins and wurtzite with a high density of stacking faults) Gallium Phosphide (GaP) nanowires (NWs) was investigated by atomic force microscopy (AFM). Experimental measurements were performed by obtaining bending profiles of as-grown inclined GaP NWs deformed by applying a constant force to a series of NW surface locations at various distances from the NW/substrate interface. Numerical modeling of experimental data on bending profiles was done by applying Euler-Bernoulli beam theory. Measurements of the nano-local stiffness at different distances from the NW/substrate interface revealed NWs with a non-ideal mechanical fixation at the NW/substrate interface. Analysis of the NWs with ideally fixed base resulted in experimentally measured Young's modulus of 155±20 GPa for ZB NWs, and 157±20 GPa for WZ NWs, respectively, which are in consistence with a theoretically predicted bulk value of 167 GPa. Thus, impacts of the crystal structure (WZ/ZB) and crystal defects on Young's modulus of GaP NWs were found to be negligible.
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