Current status and challenges of ferroelectric memory devices

We report on the state-of-the art memory devices on the basis of ferroelectric materials. The paper starts with a short survey on competitive non-volatile memory technologies and focuses then on ferroelectric memories. This includes the ferroelectric random access memory (FeRAM) and the ferroelectric field effect transistor (FeFET). Cell layouts, material aspects and CMOS compatibility as well as fabrication issues will be discussed. Beside the current research on ferroelectric memory devices we present results on the superparaelectric limit of ferroelectric materials with respect to lateral and thickness scaling. Scanning probe techniques showed ferroelectric properties in dots as small as 20 nm. Ultra thin ferroelectric films as thin as a few unit cells can be achieved on lattice matched substrates. These investigations can be considered as a guideline for the maximum achievable packaging density of FeRAMs including low power consumption. The most challenging task to achieve storage above 128 Mb, is the conformal coverage of 3-D electrodes, e.g. by atomic layer deposition (ALD). Three dimensional capacitors are mandatory to achieve sufficient charge for clear signal sensing. In addition, we present a few new concepts based on ferroresistive films, strain induced enhanced ferroelectricity, and lead-free ferroelectrics which may be relevant for the future FeRAM technology. Finally, a new challenging concept of an entire organic ferroelectric field effect transistor (OFeFET) is briefly discussed.