Charge Storage and Reliability Characteristics of Nonvolatile Memory Capacitors with HfO2/Al2O3-Based Charge Trapping Layers

Abstract

Flash memories are the preferred choice for data storage in portable gadgets. The charge trapping nonvolatile flash memories are the main contender to replace standard floating gate technology. In this work, we investigate metal/blocking oxide/high-k charge trapping layer/tunnel oxide/Si (MOHOS) structures from the viewpoint of their application as memory cells in charge trapping flash memories. Two different stacks, HfO2/Al2O3 nanolaminates and Al-doped HfO2, are used as the charge trapping layer, and SiO2 (of different thickness) or Al2O3 is used as the tunneling oxide. The charge trapping and memory windows, and retention and endurance characteristics are studied to assess the charge storage ability of memory cells. The influence of post-deposition oxygen annealing on the memory characteristics is also studied. The results reveal that these characteristics are most strongly affected by post-deposition oxygen annealing and the type and thickness of tunneling oxide. The stacks before annealing and the 3.5 nm SiO2 tunneling oxide have favorable charge trapping and retention properties, but their endurance is compromised because of the high electric field vulnerability. Rapid thermal annealing (RTA) in O2 significantly increases the electron trapping (hence, the memory window) in the stacks; however, it deteriorates their retention properties, most likely due to the interfacial reaction between the tunneling oxide and the charge trapping layer. The O2 annealing also enhances the high electric field susceptibility of the stacks, which results in better endurance. The results strongly imply that the origin of electron and hole traps is different—the hole traps are most likely related to HfO2, while electron traps are related to Al2O3. These findings could serve as a useful guide for further optimization of MOHOS structures as memory cells in NVM.

Characterization of program and erase speed of memory capacitors with nanolaminated HfO2/Al2O3 stacks for application in non-volatile memories

“…The same procedure, but with Vp =±30 V was used for the structures with thicker TO (figure 2 (b)). The applied Vp was chosen based on the previous results [9] as values for which stable and substantial charge trapping occurs and at the same time they are below the breakdown voltages. (The positive values of ΔVfb correspond to electron trapping while the negative ones represent accumulation of positive charge).…”

“…In several works we have also demonstrated the perspective of HfO2/Al2O3 stack structures, emphasizing the importance of O2 annealing for obtaining large memory windows [7][8][9]. In this study in an effort to extend the knowledge of the charge trapping characteristics of nanolaminated HfO2/Al2O3 we report experimental results on the program and erase performance characteristics.…”

Interfaces in ALD very thin Al2O3/HfO2 stacks studied by ellipsometry

“…What is unchangeable here is only the number and ratio of deposition cycles. So, we [3,6,10] measuring thicknesses of sublayers and blocks and knowing the cycle numbers plus HfO2 relative volume fractions in a block depth. The results are depicted in figure 3.…”

“…The effect of annealing on the stacks is demonstrated in figure 5. The comparison of the depth profiles in figures 2 and figure 5 suggests that annealing at 800˚C in oxygen environment for 1 min reduces the thicknesses of the sample regions in different extent [11] and increases the refractive index of the stack, while still keeping its laminated character [3] interfacial regions. Further studies are needed to evaluate the impact of inhomogeneous parts into the stack regions of the samples and their contribution to the optical constants and electrical properties.…”

“…The purpose of this layer, referred as blocking oxide (BO), is to suppress the charge injection from the gate electrode during the write stage of the memory cell and to prevent the charge loss to the gate in storage state. The deposition procedure, in more details, as well as CSTEM images of similar stacks is given in [3]. It is well known that ellipsometry is an optical, accurate and precise method for thickness measurements of thin layers, interfaces and multilayers.…”