Thursday, June 11, 2015

Samsung, NaMLab and KU Leuven present novel DRAM capacitor stack

There was a long time since I came across a publication on materials screening for the DRAM capacitor stack. That is why it is especially interesting to read of the joint work by NaMLab in Dresden , K.U. Leuven and Samsung. The team was able to enhance the properties of the high-k stack by replacing the Al2O3 interlayer with SrO to increase the overall k-value of the capacitor dielectric without degrading the barrier and leakage properties of the dielectric stack.

Introduction: For many years, the dynamic random access memory (DRAM) was the scaling driver in semiconductor industry. Continuous downscaling of the cell dimension led to the introduction of high-k materials in a three-dimensional cylindrical capacitor geometry with metal electrodes. Currently, the most common DRAM capacitor consists of a ZrO2/Al2O3/ZrO2 (ZAZ) stack. Intensive research is done on strontium titanate (STO) and Al doped TiO2 based capacitors, but here, the thickness scaling of the dielectric is difficult to reach due to the low band gap value of the TiO2 based dielectrics. Accordingly, additional research is necessary to scale the current ZrO2 based material stack.

Ultra-thin ZrO2/SrO/ZrO2 insulating stacks for future dynamic random access memory capacitor applications

Steve Knebel, Milan Pešić, Kyuho Cho, Jaewan Chang, Hanjin Lim, Nadiia Kolomiiets, Valeri V. Afanas'ev, Uwe Muehle, Uwe Schroeder and Thomas Mikolajick

J. Appl. Phys. 117, 224102 (2015);

(a) TEM micrograph of a ZAZ MIM film stack. The stack thickness is 10 nm. A distinct Al2O3 layer is visible in the center of the ZrO2 layer. ZrO2 grains stop growing at the Al2O3 interlayer. (b) TEM micrograph of a ZSrZ MIM film stack. Stack thickness is 5 nm. No distinct SrO layer is visible and the ZrO2 crystals are growing through the whole ZrO2 layer. (c) EDX line scan of the ZAZ film: a small Al peak can be seen in the center of the ZrO2 layer. (Inset) Zoom of the EDX line scan showing the Al peak and the background signal, which proves the position of the Al2O3 layer.
Citation: J. Appl. Phys. 117, 224102 (2015);

Aiming for improvement of the ZrO2-based insulator properties as compared to the state-of-the-art ZrO2/Al2O3/ZrO2 stacks beyond 20 nm dynamic random access memory (DRAM) technology applications, ultra-thin (5 nm) ZrO2/SrO/ZrO2 stacks with TiN electrodes deposited by physical vapor deposition are addressed. By replacing the Al2O3 interlayer with SrO, the effective dielectricpermittivity of the stack can be increased as indicated by electrical analysis. At the same time, no degradation of the insulating properties of the SrO-containing stacks and minor changes in the reliability, compared to an Al2O3 interlayer, are found. These results are indicating the possibility of further reducing the effective oxide thickness of the ZrO2-based stacks to come close to 0.5 nm for future DRAM capacitors.

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