(Invited) High-k Dielectrics and High Work Function Metals for Hybrid Floating Gate NAND Flash Applications
In HFG, the poly-silicon FG is replaced by a metal/Si(n-type) stack. In order to control the leakage through the dielectric placed in between FG and the control gate (also known as inter-gate dielectric, IGD), it is desired that the FG metal has a high work function (WF). The equivalent oxide thickness (EOT) of the stack is determined by the k-value of the IGD. For optimal memory performance it is required that the materials characteristics of the HFG-IGD stack remain stable throughout the device fabrication steps. Independently of the process flow, the most critical ones are the anneals performed after IGD deposition, which can be as high as 1050 °C.
We have investigated the HFG-IGD materials compatibility issues with two different type of samples: blanket films and capacitors (50x50 μm2). In blanket samples we mimic the capacitor process steps and analyze the physical properties of the HFG-IGD stack. Capacitors allow measuring the program and retention characteristics of the gate stack. The reference IGD is Al2O3 (7 nm EOT) annealed at 1000 °C. Al2O3 (k~9) in combination with TiN (WF 4.7eV) is characterized by a program window (PW) of ~4V and a charge loss of 0.08V after 24h at 85 °C (retention). However, for further scaling, larger PW for smaller EOT are required. Therefore, it is of utmost importance to use dielectrics with higher k-values as compared to Al2O3, and to search for metals with WF higher than the one of TiN.
We have investigated Ta1-xAlxNy (WF>4.9eV) and Ru (WF>4.7eV) as metal candidates; the metal of reference is TiN. The high-k IGD of choice are Hf1-xAlxOy, TiO2, Hf1-xGdxOy and (Gd,Al)1-xScxOy, with k-values ranging from ~10 to ~30. The thermal stability of the HFG stack is explored through annealing treatments performed at 500-1000 °C. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, time-of-flight secondary ions mass spectroscopy, high resolution elastic recoil detection, capacitance-voltage, current-voltage and internal photoemission measurements The impact of the stack microstructure on memory performance is discussed based on IGD composition and crystallization together with metal-IGD interdiffusion characteristics.
In general, we have observed that large program windows can be achieved even with k-values slightly higher than 10, providing that the IGD remains amorphous; k-values higher than 20 normally go along with low crystallization temperatures making these IGD’s unsuitable for our applications. The stability of the HFG-IGD stack against O, N and metal diffusion clearly influences retention. In particular, enhanced IGD crystallinity influences this diffusion behavior as well through the presence of grain boundaries. Thus, Ta1-xAlxNy and Ru are interesting cases to discuss due to the lack of stability even when combined with amorphous IGD’s. The material learning obtained from single IGD layers has allowed us to engineer them, optimizing the memory performance through the use of high-k/low-k/high-k stacked systems, paving the way to integrate HFG in 10 nm node.
This work was supported by IMEC’s Industrial Affiliation Program on Advanced Flash Memory. HR-ERD was supported by the European Community under EC SPIRIT contract 227012.
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