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During DRAM capacitor scaling a lot of effort was spent searching for new material stacks to overcome the scaling limitations of the current material stack, such as leakage and sufficient capacitance. In this study, very promising results for a SrTiO3 based capacitor with a record low capacitance equivalent thickness value of 0.2 nm at target leakage current are presented. Due to the material properties of SrTiO3 films (high vacancy concentration and low band gap), which are leading to an increased leakage current, a physical thickness of at least 8 nm is required at target leakage specifications. However, this physical thickness would not fit into an 18 nm DRAM structure. Therefore, two different new approaches to develop a new ZrO2 based DRAM capacitor stack by changing the inter-layer material from Al2O3 to SrO, and the exchange of the top electrode material from TiN to Pt are presented. A combination of these two approaches leads to a capacitance equivalent thickness value of 0.47 nm. Most importantly, the physical thickness of < 5 nm for the dielectric stack is in accordance with the target specifications. Detailed evaluations of the leakage current characteristics lead to a capacitor model which allows the prediction of the electrical behavior with thickness scaling. ispartof: Journal of Applied Physics vol:119 issue:6 pages:064101-1 status: published

The scaling of solid state memory demands an ever continuing progress in memory device concepts as well as utilization of materials with properties superior to the state of the art. Since its invention dynamic random access memory (DRAM) is one of the driving forces for the technological advance in microelectronics. However, the charge-based storage of information necessitates one large capacitor in each memory cell, posing drawbacks for further scaling. Requiring constant capacitance with decreasing capacitor area, the thickness of the dielectric has been reduced to the physical limit of only few nanometers. The utilization of high permittivity dielectrics has turned out to be the sole answer to this problem. The ongoing shrinkage of the feature size and the implicated increase of the capacitor’s aspect ratio will, however, denote a scaling limit for DRAM in the near future. As a possible successor of DRAM, but also of flash memory, redox-based resistive random access memory (ReRAM) has drawn great attention. Relying on a highly local resistive switching effect, ReRAM has the opportunity to be arranged in crossbar devices of dimensions in the low nanometer regime. However, the underlying switching mechanisms are not fully understood, requiring reliable and reproducible ReRAM memory cells for the verification of simulated and modeled switching characteristics. In this thesis high permittivity capacitors for application in DRAM as well as resistive switching devices for application in ReRAM are fabricated and characterized. To meet the highly challenging preparation and electrical characterization, both a sputter tool, optimized for ultra thin film deposition, as well as sophisticated electrical characterization techniques are developed and constructed. The memory cells are prepared using a layer by layer deposition of electrode and functional oxide films on silicon substrates. In case of capacitors for DRAM application, high permittivity strontium titanate (STO) thin films are deposited on strontium ruthenate (SRO) electrodes at elevated temperature. Capacitance equivalent thickness (CET) and leakage current are superior to the best thin film devices reported in literature with STO film thickness below 15 nm. Frequently discussed passive layers in high permittivity dielectrics are shown to be insignificant in the fabricated capacitors. The analysis of leakage mechanisms reveals a detrimental Schottky barrier reduction with decreasing film thickness. This effect leads to a tremendous increase of leakage current. The comparison of the two different material systems STO and tantalum oxide with respect to their resistive switching properties and performance is the approach for the analysis of ReRAM memory cell characteristics. The investigation of the electroforming process reveals thermal activation, a considerable relation to defect density and significant switching oxide thickness dependence. With repetitive cycling, the highly reliable switching and outstanding endurance of tantalum oxide films is demonstrated. The systematic exchange of the electrode metals exhibits a distinct electrode material dependence of resistive switching.

The electronic collections of today's libraries, museums and archives are growing and increasingly have a more relevant role in the holdings. Memory institutions must address users' need to access a widening range of digital artefacts. Often the formats of those artefacts are outdated and they cannot be run or rendered on today's systems any longer. This is where emulation can provide the required digital environments suitable for a given object type. Practical research is being done at Freiburg University for the Open Planets Foundation on how to integrate different emulators for a number of original environments into a single graphical desktop. In this case study, options for future reading room systems like stateless Linux workstations are evaluated and prototypical implementations are demonstrated.