N. Banerjee

A flexoelectric microelectromechanical system on silicon

Flexoelectricity allows a dielectric material to polarize in response to a mechanical bending moment and, conversely, to bend in response to an electric field. Compared with piezoelectricity, flexoelectricity is a weak effect of little practical significance in bulk materials. However, the roles can be reversed at the nanoscale. Here, we demonstrate that flexoelectricity is a viable route to lead-free microelectromechanical and nanoelectromechanical systems. Specifically, we have fabricated a silicon-compatible thin-film cantilever actuator with a single flexoelectrically active layer of strontium titanate with a figure of merit (curvature divided by electric field) of 3.33 MV(-1), comparable to that of state-of-the-art piezoelectric bimorph cantilevers.

Direct patterning of functional interfaces in oxide heterostructures

We report on the direct patterning of high-quality structures incorporating the LaAlO3-SrTiO3 interface by an epitaxial-liftoff technique avoiding any reactive ion beam etching. Detailed studies of temperature dependent magnetotransport properties were performed on the patterned heterostructures with variable thickness of the LaAlO3 layer and compared to their unstructured thin film analogues. The results demonstrate the conservation of the high-quality interface properties in the patterned structures enabling future studies of low-dimensional confinement on high mobility interface conductivity as well as interface magnetism.

Submicron patterning of epitaxial PbZr0.52Ti0.48O3 heterostructures

Epitaxial PbZr0.52Ti0.48O3 (PZT 52/48) heterostructures incorporating SrRuO3 electrodes were patterned through a single step epitaxial lift-off technique without using any acidic etching. This procedure enables patterning of high temperature grown perovskite multilayers through the use of a pre-patterned AlOx mask which acts as a high temperature sacrificial template. Ferroelectric properties of structured PZT grown on (001) SrTiO3 substrates as well as on commercially important platinized Si were investigated in capacitor configuration analogous to structures previously prepared through wet etching. In addition, the patterning technique is shown to be compatible with e-beam lithography for preparation of sub-micron device structures consisting of ferroelectric perovskite materials.

Fabrication of piezodriven, free-standing, all-oxide heteroepitaxial cantilevers on silicon

We report on the fabrication and mechanical properties of all-oxide, free-standing, heteroepitaxial, piezoelectric, microelectromechanical systems (MEMS) on silicon, using PbZr0.52Ti0.48O3 as the key functional material. The fabrication was enabled by the development of an epitaxial lift-off strategy for the patterning of multilayer oxide heterostructures grown on Si(001), employing a high temperature stable, sacrificial oxide template mask to obtain freestanding cantilever MEMS devices after substrate etching. All cantilevers, with lengths in the range 25–325 μm, width 50 μm, and total thickness of 300 nm, can be actuated by an external AC-bias. For lengths 50–125 μm, the second order bending mode formed the dominant resonance, whereas for the other lengths different or multiple modes were present

Modulation of Pb chemical state of epitaxial lead zirconate titanate thin films under high energy irradiation

The chemical states of epitaxial PbZrxTi1-xO3 films were investigated by an X-ray photoelectron spectroscopy as a function of the gamma-ray doses. An anomalous behaviour was observed in Pb4f states, and a core level of Pb4f shifts towards a higher binding energy at 50 kGy and towards a lower binding energy at 200 kGy. The behaviour can be explained by a radiation induced reduction of PbO to metallic Pb. The metal-insulator-metal electrodes were fabricated by lithography, and the current-voltage characteristics were measured. A negative differential resistance (NDR) was observed in the leakage currents at room temperature. A higher current and disappearance of NDR characteristics were found in the 200 kGy irradiated samples, which further confirms the presence of metallic Pb.

Optimization of piezo-MEMS layout for a bladder monitor

MEMS technology has never been used for a non-imaging wearable ultrasound application, where e.g. lateral resolution is a minor issue. We combine finite element modeling and thin-plate analytic modeling and some measurements on MEMS devices, to optimize the performance of four MEMS layouts for a wearable bladder monitor. An array of square diaphragms, clamped on two sides, appears to be most favorable.