SOI技术在MOS存储器件上的运用

1 . Lecture #29 ANNOUNCEMENTS • HW#15 will be for extra credit • Quiz #6 (Thursday 5/8) will include MOSFET C-V • No late Projects will be accepted after Thursday 5/8 • The last Coffee Hour will be held this Thursday at 5/8 • Prof. King & TAs will hold office hours through 5/22 OUTLINE • MOSFET scaling (reprise) • SOI technology • MOS memory devices Spring 2003 EE130 Lecture 29, Slide 1 Moore’s Law # transistors/chip doubles every 1.5 to 2 years 1,000,000,000 Heading toward 1 billion transistors in 2007 100,000,000 Pentium® 4 Processor 10,000,000 Pentium® III Processor Pentium® II Processor Pentium® Processor 1,000,000 486™ DX Processor 386™ Processor 100,000 286 8086 10,000 8080 8008 4004 1,000 1970 1980 1990 2000 2010 Spring 2003 EE130 Lecture 29, Slide 2 1 

2 . Intrinsic Gate Delay (CgateVDD / IDsat) 0.85V VDD=0.75V Spring 2003 EE130 Lecture 29, Slide 3 Silicon on Insulator (SOI) Technology TSOI • Transistors are fabricated in a thin single-crystal Si layer on top of an electrically insulating layer of SiO2 9 Simpler device isolation Æ savings in circuit layout area 9 Low junction capacitances Æ faster circuit operation 9 Better soft-error immunity 9 No body effect 8 Higher cost Spring 2003 EE130 Lecture 29, Slide 4 2 

3 . Partially Depleted SOI (PD-SOI) 2ε s ( 2ψ B ) TSOI > Wdm , where Wdm = qN body Floating body effect (history dependent): 1. When a PD-SOI NMOSFET is in the ON state, at moderate-to-high VDS, holes are generated via impact ionization near the drain 2. Holes are swept into the neutral body, collecting at the source junction 3. The body-source pn junction is forward biased 4. Æ VT is lowered Æ IDsat increases Æ “kink” in output ID vs. VDS curve Spring 2003 EE130 Lecture 29, Slide 5 Fully Depleted SOI (FD-SOI) 2ε s ( 2ψ B ) TSOI < Wdm , where Wdm = qN body • No floating body effect! • VT is sensitive to SOI film thickness • Poorer control of short-channel effects due to fringing electric field from drain Gate • Elevated S/D contact structure Source SOI Drain needed to reduce RS, RD SiO2 Silicon Substrate Spring 2003 EE130 Lecture 29, Slide 6 3 

4 . Semiconductor Memory • Volatile – Static random access memory (SRAM) – Dynamic random access memory (DRAM) • Non-Volatile – Mask programmed ROM – Programmable Read-Only Memory (PROM) – Electrically programmable ROM (EPROM) – Electrically erasable PROM (E2PROM) – Flash EPROM Spring 2003 EE130 Lecture 29, Slide 7 6-Transistor CMOS SRAM Cell ~1 ns read time <10 ns write time WL V DD M2 M4 Q M5 Q M6 M1 M3 BL BL Spring 2003 EE130 Lecture 29, Slide 8 4 

5 . 6T-SRAM: Layout • Modern processes can VDD fit a 6T SRAM cell in ~1.0µm2 M2 M4 Q Q M1 M3 GND M5 M6 WL BL BL Spring 2003 EE130 Lecture 29, Slide 9 SRAM Scaling Challenges • Low standby power Æ low OFF current (e.g. 1 pA/cell) Æ large VT is required • Soft error immunity Spring 2003 EE130 Lecture 29, Slide 10 5 

6 . 1-Transistor DRAM Cell ~10 ns read time BL ~100 ns write time WL Write "1" Read "1" WL M1 CS X GND VDD −VT VDD BL VDD/2 VDD /2 CBL sensing Write: CS is charged or discharged by asserting WL and BL. Read: Charge redistribution takes places between bit line and storage capacitance CS ∆ V = VBL – V PRE = ( V BIT – V PRE ) ------------------------ C S + CBL Voltage swing is small; typically around 250 mV. Spring 2003 EE130 Lecture 29, Slide 11 DRAM Cell Structure • Desired characteristics: 9 low power consumption Capacitor 9 long retention time 9 “fast” access time 9 soft error immunity • ≥25fF/cell is required for Transistor sensing signal margin and Gate retention time Drain Source Body Spring 2003 EE130 Lecture 29, Slide 12 6 

7 . Advanced DRAM Capacitor Structures Trench Capacitor Stacked Capacitor Word line Cell plate Capacitor dielectric layer Insulating Layer Cell Plate Si Capacitor Insulator Transfer gate Isolation Refilling Poly Storage electrode Storage Node Poly Si Substrate 2nd Field Oxide Spring 2003 EE130 Lecture 29, Slide 13 DRAM Scaling Challenge • Long retention time Æ low OFF current (~1 fA) – large VT is required • Fast access time Æ high ON current (~100 µA) – large (VGS-VT) is required Capacitor => VDD cannot be 4 Possible charge scaled down leakage paths aggressively, shown here: for low power consumption Gate (WL) STI 1 Spring 2003 EE130 Lecture 29, Slide 14 3 2 7 

8 . Flash EPROM Cell Structure Control Gate Inter-poly Oxide • To program this device, electrons are injected from the channel inversion layer Floating gate into the floating gate through the tunnel oxide. Tunnel Oxide N+ Source N+ Drain • The inter-poly oxide is thick, to prevent electrons from Substrate tunneling through it. • Tunnel oxide: 8 nm thermal oxide • Floating gate: 100 nm N+ poly-Si • Inter-poly oxide: 16 nm CVD oxide or Oxide/Nitride/Oxide stack Spring 2003 EE130 Lecture 29, Slide 15 Program by Hot Electron Injection +10V 0V FG +5V 3.15eV Source Drain A Floating channel Tunnel gate Substrate oxide • Electrons are accelerated by the lateral E-field and gain enough kinetic energy at point A (near the drain) to surmount the potential barrier. • Because of the control-gate bias, electrons are injected into the floating gate. Spring 2003 EE130 Lecture 29, Slide 16 8 

9 .Program by Fowler-Nordheim Tunneling +18V 3.15eV 0V FG 0V Source Drain channel A Tunnel Floating oxide gate Substrate • For a sufficiently high control-gate bias, electrons can tunnel from the channel inversion layer into the floating gate. Spring 2003 EE130 Lecture 29, Slide 17 Erase Operation -18V 0V 0V 3.15eV Source Drain channel Tunnel Floating oxide gate Substrate • Under a large negative control-gate bias, electrons tunnel out of the floating gate into the substrate. Spring 2003 EE130 Lecture 29, Slide 18 9 

10 . Sensing the Stored Data (1) Programmed state Two VT states: VT= VT2=5V, IDS=0 VR=3V IDS 0V VDS=2V Erased Programmed Source Drain 50uA Substrate (2) Erased state VT= VT1=1V, IDS=50 uA 0A VCG VT1 VR VT2 VR=3V 0V VDS=2V Source Drain Spring 2003 EE130 Lecture 29, Slide 19 Substrate NOR Flash Memory Architecture Each memory cell can be addressed individually by its word line (gate) and bit line (drain) Spring 2003 EE130 Lecture 29, Slide 20 10 

11 . NAND Flash Memory Architecture • For each bit line, 16 or 32 cells are connected, with one select transistor at each end of the bit line. • Programmed VT > 0 V Erased VT < 0 V • The source/drain region between each two adjacent cells are shared Æ high density Spring 2003 EE130 Lecture 29, Slide 21 NOR vs. NAND Architecture NOR NAND Chip Density Medium (64MB) Very high (2GB) Programming Hot electron injection F-N tunneling mechanism Programming speed 1us ~10us 1ms Erasing speed ms byte/block erase ms block erase Random access Yes No Application Code storage Data storage Vendor Intel, AMD SanDisk, Toshiba, Samsung Spring 2003 EE130 Lecture 29, Slide 22 11 

12 . Flash E2PROM Scaling Challenges hTo achieve fast programming speed and low voltage operation, the tunnel oxide thickness must be scaled down. hDefects in the tunnel oxide reduce the retention time and thereby limit the tunnel oxide Source Drain scaling, however. h Today >8nm tunnel oxide is Substrate used in commercial flash products. Spring 2003 EE130 Lecture 29, Slide 23 Semiconductor Memory Trends Capacity increases 4X every 3-4 years Today: • 1 Gb DRAM • 512 MB SRAM (2MB on-chip cache SRAM) • 1 Gb flash E2PROM Spring 2003 EE130 Lecture 29, Slide 24 12