Non-MOSFET%20Based%20Memory

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Non-MOSFET Based Memory

• Alex Rodriguez-Triana
• Terence Frederick
• April 21, 2008

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Outline

• MOSFET Based RAM Memory
• DRAM, SRAM, FLASH
• Problems with MOSFET Memory
• Scaling
• Alternative Memory
• MRAM
• FeRAM
• PCRAM
• Summary

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History of MOSFET Memory

• Concept goes back to the 1960s
• People were speculative
• BJT was more advanced and faster
• Leakage current
• They were attractive
• Simple Processing
• Layout Advantages
• Leads to high-density integrated circuits

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History of MOSFET Memory

• SRAM were proposed
• six MOSFETs per cell
• SRAM began to be used in the mid-70s
• DRAM patented in 1968
• 1 MOSFET, 1 Capacitor
• First commercial DRAM
• 1971 by Intel

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Dynamic RAM

• Most common type of RAM memory
• Arranged in a square array
• one capacitor and transistor per cell
• Stores one bit per cell
• Recharging/Refreshing capacitors lose their
  charge
• Rows Word Lines
• Columns Bit Lines

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Advantages/Disadvantages of DRAM

• Advantages
• Cost
• Small
• 1T 1C vs. 6T for SRAM
• Number of Read/Write Cycles
• gt 1015
• Disadvantages
• Slow
• Need to refresh
• Volatile
• Data is lost when memory is not powered

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Static RAM

• Memory cell uses flip-flop to store bit
• Requires 6 transistors
• Each bit is stored on 4 transistors that form two
  inverters
• Two other transistors control the access to a
  cell during read and write operations
• This storage cell has two stable states
• 0 and 1

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Advantages/Disadvantages of SRAM

• Advantages
• Performance better than DRAM
• Faster
• Less Power Hungry
• Number of Read/Write Cycles
• gt 1015
• Disadvantages
• Cost
• More than DRAM
• Volatile
• Data is lost when memory is not powered

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FLASH Memory

• Invented by Dr. Fujio Masuoka at Toshiba in 1984
• Stores information in an array of memory cells
  made from floating-gate transistors
• Single-Level Cell Devices - each cell stores only
  one bit

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Advantages/Disadvantages of FLASH

• Advantages
• Cost
• Non-Volatile
• Does not lose information when the power is off
• Low Power
• Fast Erase
• Large blocks rather than one word at a time
• Disadvantages
• Number of Read/Write Cycles
• 106
• Slow Write
• Entire block must be read, word updated, then
  entire block written back

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Future of MOSFET Memory

• Current memory technologies are nearing the end
• Main issue with MOSFET RAMs
• Scalability
• Designers put more components onto each chip
• Width of the smallest features is shrinking
• 130 nm in 2000 to 45 nm today
• Existing memory technologies will be good for
  several more generations
• Unlikely to make the transition to 22 nm
  (scheduled for 2011) or 16 nm (2018)
• New types of technologies
• MRAM, FeRAM, PCRAM

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MOSFET Scaling

• Late 1990s
• Scaling resulted in great improvement in MOSFET
  circuit operation
• Reasons for smaller MOSFETs
• Same functionality in a smaller area
• Reduces cost per chip
• Smaller ICs allow for more chips on a wafer
• Fab costs for wafer are relatively fixed

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MOSFET Scaling

• Problems when scaling too small
• Slower chip speed
• Greater delay due to interconnects
• Operational problems
• Higher sub-threshold, increased gate-oxide and
  junction leakage, lower transconductance, heat
  production, and process variation
• Simulation
• Difficult to predict what the final device will
  look like
• Modeling of physical processes
• Microscopic variations in structure due to the
  probabilistic nature of atomic processes require
  statistical predictions

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Alternative Technologies

• Magnetic RAM (MRAM)
• Ferroelectric RAM (FeRAM)
• Phase Change RAM (PCRAM)

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Magnetoresistive RAM

• Under development since the 1990s
• Data is stored by magnetic storage elements
• Formed from two ferromagnetic plates
• Plates can hold a magnetic field
• Polarization doesnt leak away with time like
  charge
• Less wear since switching states doesnt involve
  movement of electron or atoms
• One plates is a permanent magnet
• Set to a certain polarity
• Second plates field will change to match that of
  an external field
• A memory device is built from a grid of "cells"

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4MB MRAM

• 1st commercial available MRAM
• Based on 1T and 1 magnetic tunnel junction
• Isolates read and write path
• Separates programming components from the sense
  circuit
• Improved performance

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Read and Write of MRAM

• Read
• Current is passedthrough the bit
• resistance of thebit is sensed

• Write
• Current is passed through the programming lines
• Induced magnetic field is created at the
  junction, which the writable plate picks up

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MRAM

• Cell works in a toggling mode
• Same direction
• Low resistance state (0)
• Opposite direction
• High resistance state (1)

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MRAM in Embedded Systems

• Inserted late in the SC fabrication process
• Low temperature
• Compatible with CMOS processing
• Consolidate multiple MRAM into one
• highly reliable NVRAM
• Less complexity
• High performance RD/WR

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Advantages/Disadvantages of MRAM

• Advantages
• Non-volatile
• Does not require programming sequences or block
  erasing
• Very fast RD/WR and unlimited endurance
• Simple device Architecture and easy software
  development
• Due to easy write and overwrite
• Disadvantages
• Scalability of magnetic domain?
• Might have the same problems as a transistor
• Disturbance of neighboring cells when put close
  together
• Leads to false writes
• High power needed to write

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Ferroelectric RAM

• Borrows concepts from DRAM
• most popular design follows the 1T1C design
  concept
• similar/same write process
• write accomplished by applying charge that is
  stored in capacitor
• Similarity to Floating Gate Design
• 1T design
• Also reminiscent of MRAM
• focuses on ferroelectric properties, whereas MRAM
  techniques often focus on ferromagnetic
  properties
• both characteristics take form of hysteresis loop

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Structure

• 1T type
• Similar to normal transistor
• Identical to floating gate design where floating
  gate is ferroelectric material
• 1T1C type
• ferroelectric material serves ONLY as capacitor

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Recent Progress in Ferroelectric Memory
Technology

• by Hiroshi Ishiwara

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Introduction

• Two major focuses in the paper
• developing a better material to deal with leakage
  currents in 1T1C FeRAM
• replace some Fe in lattice with Mn
• Improve upon 1T FeRAM design
• create MFIS-FET
• Introduce a new 1T2C FeRAM design

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Results I
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1T2C Design

• 2 Ferroelectric capacitors of the same size
  connected to the gate of the transistor
• capacitors polarized opposite the gate
• Good performance
• non-destructive data reads
• good data retention time
• high on/off current ratio

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Advantages/Disadvantages of FeRAM

• Advantages
• lower power usage
• faster write speed
• greater number of rewrites
• already being mass-produced
• Disadvantages
• still more research to be done on reliability
  (i.e. high NRE cost)
• only applicable to a small niche

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Study of Phase Change Random Access Memory
(PCRAM) at the Nano-Scale

• by R. Zhao, L.P. Shi, W.J. Wang, H.X. Yang, H.K.
  Lee, K.G. Lim, E.G. Yeo, E.K. Chua and T.C. Chong

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Introduction

• RAM based on floating-gate design (i.e. Flash
  memory) will soon meet physical limitations
• interpoly tunneling
• intercell crosstalk
• Flash memory is the most prevalent non-volatile
  memory on the market
• a viable option must be found soon
• PCRAM may be that option

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Fabrication/Design

• Bybrid process used to etch the layers
• Electronic Beam Lithography (EBL)
• Optical Lithography
• Electrodes made of TiW
• Dielectric is common SiO2
• Phase Change material is Ge2Sb5Te2
• Feature size refers to contact between PC and
  bottom electrode

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How it Works

• Unique Phase Change material has two states
• Crystalline state has low resistance and
  represents a stored 1
• Amorphous state has high resistance and
  represents a stored 0
• To change bit from 1 to 0 (i.e. RESET), a
  relatively high voltage is applied for a short
  time such that the compound melts but is not able
  to recrystallize
• To change bit from 0 to 1 (i.e. SET), a lower
  voltage is applied for a longer time so that
  compound can crystallize

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Simulation

• Pulse generator created to produce short (lt10ns)
  signal
• Known resistance placed in circuit
• Voltages measured to determine drop across
  resistor
• Current into PCRAM approximately (V1-V2)/Rload
• Cells with feature sizes ranging from 40 to 200
  nm created
• same wafer used

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Results
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Advantages/Disadvantages of PCRAM

• Advantages
• great scalability
• fast for both reads and writes
• low current required to program
• Disadvantages
• as of yet, only in the research phase
• still limited read/write accesses (108)

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Summary
  SRAM DRAM FLASH MRAM FeRAM PCRAM
Read Speed Fast Medium Fast Fast Fast Fast
Write Speed Fast Medium Slow Fast Medium Fast
Non-Volatile No No Yes Yes Yes Yes
Endurance Infinite Infinite Limited Infinite Limited Limited
Low Voltage Yes Limited Limited Yes Limited No