Yanli Pei

MOSFET Nonvolatile Memory with High-Density Cobalt-Nanodots Floating Gate and

We report high-performance MOSFET nonvolatile memory with high-density cobalt-nanodots (Co-NDs) floating gate (the density is as high as 4-5 × 1012 /cm 2 and the size is ~2 nm) and HfO2 high-k blocking dielectric. The device is fabricated using a gate-last process. A large memory window, high-speed program/erase (P/E), long retention time, and excellent endurance till 106 P/E cycles are obtained. In addition, the discrete Co-NDs make dual-bit operation successful. The high performance suggests that high work-function Co-NDs combined with high-k blocking dielectric have a potential as a next-generation nonvolatile-memory candidate.

\hbox{HfO}_{\bf 2}

In this letter, cobalt nanodots (Co-NDs) had been formed via a self-assembled nanodot deposition. High resolution transmission electron microscopy and x-ray photoelectron spectroscopy analyses clearly show that the high metallic Co-ND is crystallized with small size of ∼2 nm and high density of (4–5)×1012/cm2. The metal-oxide-semiconductor device with high density Co-NDs floating gate and high-k HfO2 blocking dielectric exhibits a wide range memory window (0–12 V) due to the charge trapping into and distrapping from Co-NDs. After 10 years retention, a large memory window of ∼1.3 V with a low charge loss of ∼47% was extrapolated. The relative longer data retention demonstrates the advantage of Co-NDs for nonvolatile memory application.

High-k Blocking Dielectric

Tungsten nanodots (W-NDs) with an ultrahigh density of 1×1013/cm2 and a small size of around of 1.5–2 nm were successfully formed by self-assembled nanodot deposition (SAND). A metal–oxide–semiconductor (MOS) memory device was also fabricated with a W-ND layer placed between tunneling SiO2 and block SiO2. Using this device, the effects of annealing on the capacitance characteristics were investigated in detail. After 900 °C post deposition annealing (PDA), an extremely large memory window of about 9.2 V was obtained, indicating that the device is a strong contender for future nonvolatile memory (NVM) applications. The program/erase speed and retention characteristics were also evaluated. The oxidation of tungsten by oxygen from the cosputtered silicon oxide was confirmed by X-ray photoelectron spectroscopy (XPS) measurement. It is considered to degrade the retention characteristics of MOS memory devices.

Memory characteristics of metal-oxide-semiconductor capacitor with high density cobalt nanodots floating gate and HfO2 blocking dielectric

In this letter, the formation of high density tungsten nanodots (W-NDs) embedded in silicon nitride via a self-assembled nanodot deposition is demonstrated. In this method, tungsten and silicon nitride are cosputtered in high vacuum rf sputtering equipment. The W-NDs with small diameters (1–1.5 nm) and high density (∼1.3×1013/cm2) were achieved easily by controlling W composition; this is the ratio of total area of W chips to that of silicon nitride target. The metal-oxide-semiconductor memory device was fabricated with high density W-NDs floating gate and high-k HfO2 blocking dielectric. A wide range memory window (0–29 V) was obtained after bidirectional gate voltages sweeping with range of ±1–±23 V. It is feasible to design the memory window with propriety power consumption for nonvolatile memory application.

Electrical Characterization of Metal–Oxide–Semiconductor Memory Devices with High-Density Self-Assembled Tungsten Nanodots

In this letter, tungsten nanodots (W-NDs) in silicon nitride formed by a self-assembled nanodot deposition method have been investigated as a floating gate of nonvolatile memory (NVM). Observations from transmission electron microscopy and x-ray diffraction pattern clearly confirm the formation of crystallized W-NDs with a diameter of ∼5 nm. The metal-oxide-semiconductor device with W-NDs in silicon nitride exhibits a larger memory window (∼4.1 V at ±12 V sweep), indicating charge trapping and distrapping between the W-ND and a silicon substrate. The program/erase behaviors and data retention characteristics were evaluated. After 10 years retention, a large memory window of ∼3.4 V with a low charge loss of ∼15% was extrapolated. These results demonstrate advantages of W-NDs in silicon nitride for the NVM application.

Formation of high density tungsten nanodots embedded in silicon nitride for nonvolatile memory application

We studied the effects of postdeposition annealing (PDA) on the films of cobalt nanodots (Co-NDs) dispersed in silica formed by self-assembled nanodot deposition (SAND). High-resolution transmission electron microscopy (HRTEM) analysis showed that the as-grown Co-NDs have a high density of 8×1012/cm2 and a small size of ~1.5 nm. After PDA at 800 °C, a monolayer of Co-NDs is produced by agglomeration. Under this PDA condition, the dot size and density are easily controlled by adjusting the thickness of the as-grown Co-ND film. In contrast, a high-temperature PDA of 900 °C induces the diffusion of cobalt into the silicon substrate and leads to the failure of memory effect. When the PDA temperature is between 600 and 800 °C, a large counterclockwise hysteresis memory window is obtained. Furthermore, in this region, the charge retention is enhanced by increasing the PDA temperature, which presumably contributes to the release of oxygen from oxidized cobalt.

Memory characteristics of self-assembled tungsten nanodots dispersed in silicon nitride

Metal-oxide-semiconductor field-effect transistor (MOSFET) nonvolatile memories with high-density tungsten nanodots (W-NDs) dispersed in silicon nitride as a floating gate were fabricated and characterized. The W-NDs with a high density of ~5 × 1012 cm−2 and small sizes of 2–3 nm were formed by self-assembled nanodot deposition (SAND). A large memory window of ~1.7 V was observed with bi-directional gate voltage sweeping between −10 and +10 V. Considering that there is no hysteresis memory window for the reference sample without W-NDs, this result indicates the charge trapping in W-NDs or related defects. Finally, the program/erase speed and retention characteristics were investigated and discussed in this paper.

Effects of Postdeposition Annealing on Cobalt Nanodots Embedded in Silica for Nonvolatile Memory Application

Tungsten Nanodots Floating Gate and HfO2 Blocking Dielectric Yanli Pei, Masahiko Nishijima, Takafumi Fukushima, Tetsu Tanaka and Mitsumasa Koyanagi International Advanced Research and Education Organization, Tohoku University 6-6-03 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan Phone: +81-22-795-4031, E-mail: sdlab@sd.mech.tohoku.ac.jp 2 Institute for Materials Research, Tohoku University Dept. of Bioengineering and Robotics, Graduate School of Engineering, Tohoku University

MOSFET nonvolatile memory with a high-density tungsten nanodot floating gate formed by self-assembled nanodot deposition

High Density Self-Assembled Tungsten Nano-dot Yanli Pei, Takafumi Fukushima, Tetsu Tanaka, Mitsumasa Koyanagi 1 International Advanced Research and Education Organization, Tohoku University, 6-6-03 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan 2 Dept. of Bioengineering and Robotics, Graduate School of Engineering, Tohoku University 6-6-01 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Phone: +81-22-795-6906, E-mail: sdlab@sd.mech.tohoku.ac.jp