NanostructuresNanostructures forfor TeraTera--bitbit LevelLevel ChargeCharge TrapTrap FlashFlash MemoriesMemories

Byung-Gook Park, Il Han Park, Jung-Hoon Lee, Gil Sung Lee, Jang-Gn Yun Inter-University Semiconductor Research Center School of Electrical Eng. and Computer Sci. Seoul National University

NanoForum'09 Outline

I. Introduction II. NAND Cell Structure III. NOR Cell and Array Structure IV. AND Cell and Array Structure V. STAR NAND Flash Structure VI. Conclusions

NanoForum'09 SNU SoEECS & ISRC and Mobile Equipments

32

NanoForum'09 SNU SoEECS & ISRC Expedited Growth Theory - NAND Flash

100 ITRS (2008) 1000

Hwang’s law Capacity (Giga Bit)

100

10

Technology Node (nm) 10 2002 2004 2006 2008 2010 2012 2014 2016 2018 Year of Development q Expedited growth theory of NAND flash memories è Year 2011 1Tb capacity with 20nm feature size

NanoForum'09 SNU SoEECS & ISRC Hard Disk Drive and Flash Memory

NanoForum'09 SNU SoEECS & ISRC Growth of Storage Capacity

HDD 10T ODD FGF 1T CTF 100G FeRAM 10G MRAM PRAM 1G 100M 10M 1M Capacity (bit) 100k 10k 1k 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 Year

NanoForum'09 SNU SoEECS & ISRC Floating Gate vs. Charge Traps

Floating Gate SONOS structure ONO structure Composite Ø No floating gate Dielectrics Gate - FG-FG space Gate - FG-active space - Single gate structure n+ n+ n+ n+ P-Si P-Si Ø Defect immunity - Non-conductive trap layer Tunnel Blocking Poly Si Si SiO2 Si3N4 SiO2 - Discrete trap storage 1.05 3.1 3.1

e e e Ø 3D structure compatibility h h h 8.0 - Insulating storage node 3.8 3.8 - Simple fabrication 1.85

NanoForum'09 SNU SoEECS & ISRC Outline

I. Introduction II. NAND Cell Structure III. NOR Cell and Array Structure IV. AND Cell and Array Structure V. STAR NAND Flash Structure VI. Conclusions

NanoForum'09 SNU SoEECS & ISRC Arch Structure (1)

q Utilization of curved surfaces for field enhancement - fast “program”and “erase”

- increased effective area r2

r1

Poly

ONO Arch type Planar type 12.0M 10.0M SiO2 8.0M 6.0M 4.0M Bottom 2.0M Top Oxide Oxide Si Electric Field (V/cm) 0.0 0 2 4 6 8 10 12 14 Position (nm) NanoForum'09 SNU SoEECS & ISRC Arch Structure (2)

q Fabrication procedure

Hard mask

Si Si SiO2

Si

NanoForum'09 SNU SoEECS & ISRC Arch Structure (3)

q Utilization of HSQ mask characteristic q Planarization by TEOS, HSQ and etch back

NanoForum'09 SNU SoEECS & ISRC Arch Structure (4)

0.5 5.5 0.0 5.0 -0.5 4.5 -1.0 4.0 -1.5 3.5 -2.0 3.0 -2.5 -18V

(V) 2.5 -3.0 (V)

th -20V 2.0 th -3.5 V 20V V -22V D

1.5 D -4.0 1.0 22V -4.5 -24V 0.5 24V -5.0 0.0 -5.5 -0.5 -6.0 -7 -6 -5 -4 -3 -2 -1 0 1 Initial10 10 10 10 10 10 10 10 10 Initial10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 Time(s) Time(s)

q Radius of Si channel = 15 nm

NanoForum'09 SNU SoEECS & ISRC Outline

I. Introduction II. NAND Cell Structure III. NOR Cell and Array Structure IV. AND Cell and Array Structure V. STAR NAND Flash Structure VI. Conclusions

NanoForum'09 SNU SoEECS & ISRC Cone Structure (1)

q Utilization of field and current concentration - field concentration in the horizontal direction - current concentration in the vertical direction

Cut line Drain junction Cut line Drain junction

ONO layer Gate Gate ONO layer Electric field

high flux density high flux density r2 flux Si r1 Drain O O ER Er N Source Gate low flux density low flux density

NanoForum'09 SNU SoEECS & ISRC Cone Structure (2)

q Simple array structure - common source architecture - word line connection through small spacing of cones

NanoForum'09 SNU SoEECS & ISRC Cone Structure (3)

NanoForum'09 SNU SoEECS & ISRC Cone Structure (4)

q Electrical characteristics

4.0 4.0 o CHEI Program( V : 6 V, V :4 V) Retention Characteristics(150C) 3.5 G D 3.5 program state

3.0 3.0

CHEI Program( V : 6 V, V :3 V) 2.5 G D 2.5

(V) 2.363 V

2.0 2.0 2.160 V ( V) TH TH V 1.5 V 1.5 1.0 1.0 FN Erase ( V : 11 V) 0.5 Erase state 0.5 B

0.0 0 1 2 3 4 5 6 7 8 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0.01 0.1 1 10 10 10 10 10 10 10 10 10 10 Time (s) Time (s)

NanoForum'09 SNU SoEECS & ISRC Outline

I. Introduction II. NAND Cell and Array Structure III. NOR Cell and Array Structure IV. AND Cell and Array Structure V. STAR NAND Flash Structure VI. Conclusions

NanoForum'09 SNU SoEECS & ISRC Conventional Flash Memory Structures

< NAND > < NOR > < AND >

LSL LBL Active

W/L

F-Poly

Schematic Top View

NAND NOR AND program efficiency high Low high sensing speed low high high density high low low

NanoForum'09 SNU SoEECS & ISRC Vertical AND Structure (1)

q device with vertical and double gate structures - vertical structure, S/D junctions connected by diffusion layer à High integration density. - double gate structure. à High device performance, high sensing speed.

N N N

P P P

N N N

P

NanoForum'09 SNU SoEECS & ISRC Vertical AND Structure (2)

q Fabrication procedure

N N N N N N

P P P P

N N N N

P P N N P N P N N N N N N N N N N N

P P P P P P P P P P

N N N N N N N N N N

P P P P (a) (b) (c) (d)

N N

N N N N

P P P P

N N N N

N P N P N P N P N N N N N N N N N N N N

P P P P P P P P P P P P

N N N N N N N N N N N N

P P P P (h) (g) (f) (e)

NanoForum'09 SNU SoEECS & ISRC Vertical AND Structure (3)

q Program/erase characteristics

7 7

V = 13 V 6 G VG = 15 V 6

VG = 17 V 5 5

4 4 V = 13 V G V = 15 V 3 G V = 17 V 3 G 2 Threshold Voltage [V] Threshold Voltage [V] 2 -5 -4 -3 -2 -1 0 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 10 10 10 10 10 Programming Time (sec) Erasing Time (sec)

NanoForum'09 SNU SoEECS & ISRC Outline

I. Introduction II. NAND Cell Structure III. NOR Cell and Array Structure IV. AND Cell and Array Structure V. STAR NAND Flash Structure VI. Conclusions

NanoForum'09 SNU SoEECS & ISRC STAR NAND Flash Structure (1)

WL direction q Stacked bit-lines gate à high density

oxide q Cylindrical channel and gate ONO dielectrics gate-all-around cell Si structure cylindrical channel à high performance

Si body

oxide q Single-crystal Si channel à high performance, uniformity, reliability Si substrate

NanoForum'09 SNU SoEECS & ISRC24 STAR NAND Flash Structure (2)

q Fabrication procedure

mask a > b nitride nitride nitride SiGe SiGe SiGe Si Si Si SiGe SiGe SiGe

Si Si Si

SiGe SiGe SiGe

Si substrate Si substrate Si substrate

SiGe SiGe nitride nitride nitride a > b

SiGe Si Si Si

SiGe Si Si Si

SiGe

Si substrate Si substrate Si substrate

NanoForum'09 SNU SoEECS & ISRC STAR NAND Flash Structure (3)

q Fabrication procedure (continued)

WL direction

gate gate SiGe

ONO nitride oxide

gate ONO dielectrics Si Si Si Si

cylindrical channel

Si Si Si body

ONO oxide

Si substrate Si substrate Si substrate

NanoForum'09 SNU SoEECS & ISRC STAR NAND Flash Structure (4)

q Components of stack and nanowire implementation

NanoForum'09 SNU SoEECS & ISRC27 Conclusions (1)

q Charge trap flash memory including SONOS structure is a promising candidate for the next generation high density flash memories.

q For NAND application, arch SONOS flash memory is proposed for field concentration and suppression of back tunneling and is successfully demonstrated.

q For NOR application, cone SONOS flash memory is proposed for field and current concentration, and the fabricated cell shows superb electrical characterics.

NanoForum'09 SNU SoEECS & ISRC Conclusions (2)

q For AND application, vertical AND structure is proposed for drastic reduction of cell size and the feasibility is demonstrated.

q For further increase of density, STacked ARray (STAR) NAND array is proposed.

NanoForum'09 SNU SoEECS & ISRC