Scaled Planar Floating-Gate NAND Flash Memory Technology:
Problems and Novel Solutions
Shyam Raghunathan
Advisor: Prof. Krishna Saraswat
Co-Advisors: Prof. Yoshio Nishi, Prof. Tejas Krishnamohan
Chair: Prof. Butrus Khuri-Yakub
Date: Sept 17th, Friday, 10.30 am (Refreshments at 10.15 am)
Venue: Allen CIS-X Auditorium
Flash memory is the most widely used non-volatile information-storage
technology today. NAND Flash memories are ubiquitous in their use as
portable storage media in cellphones, cameras, music players, and
other portable electronic devices. In addition, NAND Flash memory has
recently seen rapid adoption as Solid-state drives (SSD) in place of
Hard-disk drives (HDD) in modern personal computers and data servers.
In addition to greater speed, SSDs also provide much lower power
consumption compared to HDDs.
The NAND Flash memory device, consisting of a floating-gate
transistor cell, is the most aggressively scaled electronic device, as
evidenced by ever-increasing memory capacities. In this talk, we will
examine some problems in the continued scaling of these structures and
discuss novel solutions to overcome them.
(1) Firstly, we investigate scaling of the conventional poly-silicon
floating-gate, aimed at reducing cell-to-cell interference. We
demonstrate experimentally a new reliability concern for the first
time, arising due to programming current becoming increasingly
ballistic through ultra-thin poly-silicon floating-gates. We also
experimentally demonstrate doping-related issues in the poly-silicon
floating-gate.
(2) We then demonstrate a novel metal-based floating-gate cell for
the first time, designed to overcome the problems discussed above. We
explore factors that influence the choice of metal and we demonstrate
excellent functionality in ultra-thin metal floating-gate cells scaled
down to 3 nm TiN floating-gate thickness, thus greatly reducing
cell-to-cell interference.
(3) Finally, in order to facilitate continued scaling of the control
dielectric, we explore replacement of the conventional Oxide-Nitride
dielectric with high-k dielectric materials. We demonstrate
integration of both poly-silicon and metal floating-gate cells with
Al2O3 high-k control dielectric, thereby enabling the planar
floating-gate cell.
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