FeRAM

The ferroelectric random access memory (FeRAM), which makes use of an integrated ferroelectric (FE) thin film capacitor to store data, is one of the most promising nonvolatile (NV) memory devices [1, 2]. Key parameters for the scaling of ferroelectric random access memories are shrinking of the feature size, reduction of operation voltage, and enhancement of voltage sensing. In this chapter, we will summarize several key technology issues which have been addressed by research groups worldwide in order to develop high-density integrated NV-FeRAMs. The first section briefly deals with the principles of FeRAM and covers some physical properties of FE thin films. FeRAM utilizes the spontaneous polarization distinctive of FEs to store binary information. An external field applied to read-out the stored information may switch or not switch the spontaneous polarization apparent in a larger or smaller amount of charge on the sense amplifier, respectively. The second section covers trends in FeRAM integration. The limits of planar FE capacitor stacks lead directly to the requirement for three-dimensional (3D) capacitor structures, where severe limitations of the metal organic chemical vapor deposition (MOCVD) technique will become apparent. The third section explains the strategy for uniformly coating 3D nanostructures with FE thin films by atomic layer deposition (ALD). In contrast to binary metal oxides for gate dielectrics, FE materials are multicomponent oxides. Their growth therefore requires a combination of the ALD processes of the respective component binary oxides. The fourth section comprises the efforts which were undertaken to develop an ALD-type process for the multicomponent FE oxide Pb(Zr_xTi_1-x)O₃ (PZT). Special attention is given to the growth of stoichiometric and conformal layers into 3D capacitor hole structures. The chapter closes with recent trends leading to ALD PZT in ferroelectric field effect transistor (FeFET) devices and ALD of alternative FE thin film materials.