Formation and characterization of SrBi 2 Ta 2 O 9 (SBT)thin film capacitor module with Platinum-Titanium bottom and Platinum top electrodes

摘要

New ferroelectric memories (FeRAMs) provide a enormous business potential. They are nonvolatile, they can be designed like a DRAM with a small cell, and they exhibit fast read and write times. Especially interesting for mobile applications are FeRAMs with SrBi2Ta2O9 (SBT) due to their low voltage and low power behavior. Although standard CMOS processes can be used for front-end and back-end processes, FeRAM technology development has to overcome major challenges due to new materials used for the capacitor formation like SBT and Platinum. The main process steps for the formation of the capacitor module are (A) building of the bottom electrode and its interaction with the ferroelectric layer, (B) the deposition and crystallization of the ferroelectric layer, (C) the structuring of the top electrode and the recovery annealing after the patterning of the top electrode, and (D) the back-end processes like interlayer dielectric deposition and forming gas anneal and its impact on the performance of the ferroelectric capacitor. The objective of the thesis is to analyze all these processes and to characterize the interactions of the Pt/SBT/Pt stack during bottom electrode processing, SBT layer annealing, top electrode patterning and hydrogen annealing. Models are developed to explain the observed results. (A) It is shown that by using Pt/TiOx bottom electrodes higher polarization values compared to Pt/Ti bottom electrodes are obtained. By using Total X-Ray Fluorescence (TXRF) technique, the loss of bismuth into the Pt electrode is found to be almost 40% higher for Pt/Ti electrodes compared to Pt/TiOx electrodes. This difference in Bi-loss results in different crystallization of SBT at the Pt-SBT interface. For the Pt/Ti electrode the formation of Bi-deficient SBT with a lower dielectric constant at the interface could explain the higher polarization values for Pt/TiOx electrodes compared to Pt/Ti. The critical parameters for both hillock formation on Pt bottom electrode and for Pt adhesion are also discussed. (B) The effects of crystallization of SBT at different temperatures and at different film thickness are analyzed. The higher the crystallization temperature the larger the SBT grains and the higher the polarization values. The thinner the SBT films the more secondary phases are observed. In addition the results of the crystallization of SBT under low-pressure and in nitrogen is presented with the focus on Bi-loss. (C) Electrical and analytical results of patterned capacitors with both crystalline and non-crystalline SBT are shown. It is revealed that after patterning and annealing of the Pt/SBT stack, a degradation of the remanent polarization and leakage current for smaller feature sizes is observed. No degradation of electrical properties is seen after etching of non-crystalline SBT capacitors and annealing. A model to explain this difference of the electrical properties between crystalline and non-crystalline SBT capacitors is introduced. The model is based on a damaged zone with a Bi deficiency at the edge of the capacitor after patterning. (D) The effects of annealing in forming gas (FGA: 5% hydrogen, 95% nitrogen) on SBT films on platinum bottom electrode and on silicon dioxide with and without platinum top electrode are studied. The films were characterized by residual stress measurements, Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES), High Temperature X-Ray Diffraction (HT-XRD) and Secondary Ion Mass Spectrometry (SIMS). After FGA of blanket SBT films, tall platinum-bismuth whiskers are seen on the SBT surface. FGA of the entire Pt/SBT/Pt/Ti stack shows two different results. For the samples with a high temperature annealing step in oxygen after top electrode patterning, peeling of the top electrode is observed after FGA. For the samples without a high temperature annealing step, no peeling is observed after FGA. The findings are discussed as a results of stress behavior and by the reduction of BiOx to metallic Bi by the hydrogen.