Nonvolatile Memory Characteristics of NMOSFET with Ag Nanocrystals Synthesized via a Thermal Decomposition Process for Uniform Device Distribution
Abstract—
This paper presents nonvolatile memory characteristics using Ag nanocrystals (NCs) formed by a thermal decomposition and size-selective precipitation technique for Flash memory application. In the NC formation process, the size of NCs and the space NC-to-NC were precisely controlled by a size-selective precipitation technique and the length of the self-assembled monolayer surrounding the NCs, respectively. The size and density of the Ag NCs synthesized were typically 3-5 nm and 2.7 × 1012 cm-2, respectively. Due to the regularly distributed Ag NCs with high density, uniform memory characteristics and high program efficiency were achieved from NMOSFETs embedded with the Ag NCs, which were fabricated by the gate-last process.
Index Terms—nonvolatile memory, flash, thermal
decomposition process and size-selective precipitation technique,
metal nanocrystals.
I. INTRODUCTION
HE rapid growth of the mobile device market has lead to a
need for high-density and high-performance nonvolatile
memory (NVM) devices for better bit density and bit cost.
With Flash memory, as the device shrinks below 45 nm, the
nonvolatile characteristics are severely degraded due to the
ultimately scaled gate dielectric thickness. Moreover, as
memory cells are more closely packed, there are barriers to normal operation by an interference effect among adjacent
cells.
Fig. 1. Nonvolatile memory schematics of a nanocrystal embedded structure.
As such, nanocrystal nonvolatile memory (NC NVM) has
predominant advantages when it was proposed in the early
1990s by Tiwari [1]. By using laterally uncoupled NCs as a
floating gate, high immunity is guaranteed for stress-induced
leakage current (SILC) and dielectric defects. The only stored
charges at the nanocrystals adjacent to the defect leak through
the tunneling dielectric, compared to huge charge loss of
conventional Flash memory due to the lateral charge transport.
This signifies that the NC NVM (Fig. 1) has ability to alleviate
the scaling limitations of conventional Flash memory as well as
extended the retention time [2],[3]. Additionally, NC NVM can
effectively minimize the cross-talk effect among adjacent cells
by reducing the parasite comparing to the conventional Flash
memory.
As mentioned above, NC NVM has ability to enhance the
immunity from defects and to reduce the extrinsic NVM
characteristic deviation by means of the cross-talk effect.
However, there are key issues to be considered in the formation
of NCs. The first consideration is that the distribution of NCs
can intrinsically affect the characteristics of the NC NVMs. In
the tens of nanometer scale regime, the memory window
distribution can be scattered by irregularly distributed NCs. In
addition, the narrowed NC-to-NC space by badly ordered NCs
can potentially cause a lateral charge transport by direct
tunneling and trap-assisted (Frenkel-Poole) tunneling,
especially under the drain disturb condition.3

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