Nanometer-sized etching of lithium niobate domain wall devices

Abstract

Ferroelectric LiNbO3 single crystals have piezoelectric, pyroelectric and electro-optic properties and are widely used in nonvolatile memories, sensors, and electro-optic modulators. The material is hard and relatively inert for dying etching using thick Cr/Ni mask layers which are incompatible with the CMOS manufacturing technologies. Moreover, the etching angles are generally below 80° that could result in poor polarization retention and a large imprint effect. Here we developed an etching technique on the basis of the solid-state reaction and wet solution corrosion together that used CMOS-compatible SiO2 masks to fabricate large arrays of LiNbO3 nanodevices in high precision. The LiNbO3 nanopatterns at the regions beyond the SiO2 masks can react with the deposited Al metal below 500 °C in formation of amorphous layers in targeted thicknesses that vary with the annealing temperature and time at a reduced atmosphere. The amorphous layers can be dissolved in mixed solutions of H2O, NH4OH, and H2O2 at 85 °C, and the etching angles of memory cells in lateral sizes of 20–220 nm can approach 90° after the passivation of the LiNbO3 surface layer. The transmission electron microscope study shows the oxygen deficiency at the surface. After the connection of the cell to two side electrodes, domain switching occurs under an applied horizontal electrode field along the polar Z-axis accompanying the formation of conducting domain walls against the peripheral unswitched domain at the substrate. The read current can highly reach the level of 2–20 μA in high reliability. This etching technique promotes the large-scale fabrication of LiNbO3 nanodevices on the Si platform.