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Magnetic Random Access Memory (MRAM). Noah Flaks, Patti Viri, Jeremy Yim, Tara Zedayko Cornell University Materials Science and Engineering MS&E 407 11/02/2005. Contents. General electromagnetic concepts Competing Technologies Market History of MRAM Development

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Magnetic random access memory mram

Magnetic Random Access Memory (MRAM)

Noah Flaks, Patti Viri, Jeremy Yim, Tara Zedayko

Cornell University

Materials Science and Engineering

MS&E 407

11/02/2005


Contents
Contents

  • General electromagnetic concepts

  • Competing Technologies

  • Market

  • History of MRAM Development

  • What MRAM is and how it works

  • Challenges

  • Manufacturing / Fabrication

  • Major MRAM Competitors

  • References


Magnetoresistance
Magnetoresistance

http://www.stoner.leeds.ac.uk/research/gmr.htm


Ferromagnetism
Ferromagnetism

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html#c1


DRAM

  • DYNAMIC random access memory

  • Slow

  • Needs to be refreshed, hence dynamic


FRAM

  • Structurally similar to DRAM

  • Ferroelectric film made of PZT

  • Nonvolatile

  • Fast read/write times


SRAM

  • STATIC random access memory

  • Faster, more reliable than DRAM

  • Need not be refreshed, hence static

  • Expensive, used in CPU cache


Flash memory
Flash Memory

  • Non-volatile

  • NOR and NAND

  • Does not need to be refreshed


Comparison of ram
Comparison of RAM

Slaughter J.M et al; Fundamentals of MRAM Technology; Journal of Superconductivity; 15(1), 19; 2002


The market for mram
The Market for MRAM

  • Technology as-is: Niche markets

    • Weapons, Defense

  • The near future: Portable Electronics

    • Speed

    • Data reliability

    • Low power

    • High-performance writes

    • Unlimited read-write endurance

    • Low write energy

www.freescale.com


A brief history of mram cross tie random access memory cram
A Brief History of MRAM:Cross-Tie Random Access Memory (CRAM)

  • 1982: Naval Surface Weapons Center

  • First technology to use a magnetic element for storage

    • Same element used for magnetoresistance (MR) readout

      • 81-19 Ni-Fe (Permalloy): 400 Å

  • Obstacles

    • Consistent writing

    • Low signal

Schwee et al, J. Appl. Phys, 53(3), 1982, 2762.


A brief history of mram anisotropic magnetoresistance amr
A Brief History of MRAM:Anisotropic Magnetoresistance (AMR)

  • Mid 1980’s: Honeywell

    • Writing using magnetic hysteresis

    • Reading using MR of the same body where the data is stored

    • Memory cells integrated on an IC chip

    • Cobalt-permalloy alloy

      • AMR 2%

  • Changes in electrical resistance as magnetization is altered

    • Thin films

      • magnetization can be single domain

      • Demagnetizing fields force magnetization direction to remain virtually parallel to the plane of the film.

      • Maximum resistance when magnetization and current are parallel

Daughton, J. Magnetoresistive Random Access Memory, 2000


A brief history of mram giant magnetoresistance gmr
A Brief History of MRAM:Giant Magnetoresistance (GMR)

  • 1988: (001)Fe/(001)Cr Magnetic Superlattices

  • Multilayers experience a dramatic change in electrical resistance under the influence of varying external fields.

    • MR = 6%

  • Read times < 50 ns

  • Limitations

    • Semiconductor still faster

      • Low MRAM sense signal

    • Cell size

      • Sense lines > 1 µm for write integrity.

Baibich et al, Phys. Rev. Lett., 61(21), 1988, 2473.


A brief history of mram pseudo spin valves psv

1996: Nonvolatile Electronics

Significant improvement of signal levels

0.2 µm memory cells – Dense

Mismatched film properties achieved by varying thickness

Thin = “soft” = lower field for switching

Means of reading storage state

Thicker = “hard” = higher field

Storage Layer

A Brief History of MRAM:Pseudo-Spin Valves (PSV)

Daughton, J. Magnetoresistive Random Access Memory, 2000


A brief history of mram spin dependent tunneling sdt memory aka magnetic tunnel junction mtj
A Brief History of MRAM:Spin Dependent Tunneling (SDT) MemoryAKA Magnetic Tunnel Junction (MTJ)

  • Quantum tunneling through a thin insulator between two magnetic layers.

    • Aluminum Oxide

    • 15 Å

  • MR > 40%!

  • Neither magnetic film “pinned”

Daughton, J. Advanced MRAM Concepts, 2001.


Mram how it works what is mram

Magnetoresistive Random Access Memory (MRAM) stores data utilizing the magnetic polarity of a ferromagnetic layer.

Current is determined by the rate of electron quantum tunneling through the MRAM stack, which is affected by the polarity of the cells.

Resistance is measured across the stack to determine the cell state. The free layer polarization is changeable: thus parallel or antiparallel magnetic moments give low or high resistances, 23σ difference, which can be interpreted as “0” or “1.”

MRAM: How it WorksWhat is MRAM?

www.freescale.com

Durlam et al, IEDM Tech Dig, 2003.


Mram how it works freescale 4mb mram die
MRAM: How it Works utilizing the magnetic polarity of a ferromagnetic layer.Freescale 4Mb MRAM die

  • 0.18 µm CMOS with 3 layers of aluminum and 2 layers of copper interconnects

  • Cladded write lines

  • 256Kb x 16 organization

  • 3.3V supply voltage

  • Symmetrical 25ns read and write timing

  • Bit cell size = 1.55µm2

  • Die size 4.5 x 6.3mm

www.freescale.com


Mram how it works reading and writing

The sense circuit (black lines) is composed of a thin oxide pass transistor, which is connected to the MTJ by the top and bottom electrodes.

Writing to the MRAM bit is done by accessing write lines 1 and 2 (red lines) in a series of pulses designed alter the state of the free layer. This process is known as Savtchenko switching, which will be discussed later.

Reading and Writing to an MRAM bit are accomplished through different circuits. Separate circuits for reading and writing reduces number parasitics improves operation speed.

MRAM: How it Works Reading and Writing

Durlam et al, IEDM Tech Dig, 2003.


Mram how it works mram has layers
MRAM: How it Works pass transistor, which is connected to the MTJ by the top and bottom electrodes.MRAM has layers

  • The free layer is in fact a Synthetic Antiferromagnetic tri-layer stack (SAF).

  • The magnetic moments of the top and bottom layers are nearly balanced. The direction of the top FM layer and the sense FM layer are set by Savtchenko switching.

  • Direction of magnetization of the ferromagnetic (FM) sense layer with respect to the pinned FM layer determines the resistance state of the bit.

www.freescale.com

Slaughter, CNS Symposium, 2004.


Mram how it works savtchenko switching
MRAM: How it Works pass transistor, which is connected to the MTJ by the top and bottom electrodes.Savtchenko Switching

  • Named after the late Leonid Savtchenko at Motorola.

  • Savtchenko switching is a method to toggle bit between high and low resistance states.

  • The SAF rotates its magnetic axis perpendicular to the applied field. The bit is oriented 45 degrees with respect to the write lines.

  • 45 degree bits results in higher memory storage densities.

Durlam et al, IEDM Tech Dig, 2003.


Mram how it works savtchenko continued

Hard pass transistor, which is connected to the MTJ by the top and bottom electrodes.

Hard

Hard

Hard

Hard

Hard

Hard

Hard

Hard

Hard

Hard

Hard

Hard

Hard

Hard

Easy

Easy

Easy

Easy

Easy

Easy

Easy

Easy

Easy

Easy

Easy

Easy

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Axis

Write Line 1

Write Line 1

Write Line 1

Write Line 1

WriteLine1

H

H

2

2

Write Line 2

Write Line 2

Write Line 2

Write Line 2

Write Line 2

I

I

2

2

I

I

H

H

1

1

1

1

Write Line 1

On

Off

On

Write Line 2

Off

t2

t1

t3

t4

t0

MRAM: How it Works Savtchenko Continued

  • Pulses are applied in a sequence designed to rotate the SAF 180 degrees to the opposite resistance state. Note that other MRAM cells are not affected during this writing process, because a single line alone cannot switch a bit.

Slaughter, CNS Symposium, 2004.


Mram how it works overlapping pulse switches
MRAM: How it Works pass transistor, which is connected to the MTJ by the top and bottom electrodes.Overlapping Pulse Switches

  • Method allows for virtually no “disturbs:” writing to one cell does not alter the state of another.

  • Requires overlapping pulse for switching.

  • Better than conventional method because in ½ selected distributions all bits along selected lines have a reduced energy barrier during programming.

    • Susceptible to disruptions from thermal agitation.

Slaughter, CNS Symposium, 2004.


Challenges in manufacturing and fabrication
Challenges in Manufacturing and Fabrication pass transistor, which is connected to the MTJ by the top and bottom electrodes.

  • Reducing drive currents

  • Avoiding thermal instabilities

  • Increasing density of MRAM array


Challenges cont d
Challenges – cont’d pass transistor, which is connected to the MTJ by the top and bottom electrodes.

Smith, J. Magnetic RAM at Freescale Semiconductor, 2004

  • Reducing Drive Currents:

    • Lower power use desired

    • Solution: Coat 3 edges of the conducting strip lines with cladding made of ferromagnetic material

      • Cladding concentrates the magnetic field toward the magnetic stack stack

      • Less current is needed to achieve the same magnetic field

Daughton, J. Magnetoresistive Random Access Memory, 2000

Durlam et al, IEDM Tech Dig, 2003.


Challenges cont d1
Challenges – cont’d pass transistor, which is connected to the MTJ by the top and bottom electrodes.

  • Avoiding thermal instabilities

    • As size decreases, barrier energy drops as well, decreasing stability. Cannot just raise Hc, because increases current.

    • Solution: use heat to help select cell for writing

      • As material approaches Curie point, Hc drops, so that less current is needed to write

      • At cooler temps, the energy well can be deeper, increasing stability.


Challenges cont d2
Challenges – cont’d pass transistor, which is connected to the MTJ by the top and bottom electrodes.

  • Increasing density of MRAM array

    • Limited by density of semiconductor technology

    • Thermal incompatibilities between magnetic and semiconductor materials. Semiconductor materials processing requires high temperatures, at too high temperatures, magnetic materials lose their properties.

    • Solution: Laser annealing

      • Can adjust heating/cooling rate to affect structure

      • Shallow thermal penetration depth should prevent damage to magnetic materials.


Manufacturing fabrication
Manufacturing / Fabrication pass transistor, which is connected to the MTJ by the top and bottom electrodes.

  • Lab-Fab approach allows Freescale to achieve manufacturing goals:

    • Lowest cost options

    • Limit risk

    • Fastest time to market


Major mram competitors
Major MRAM Competitors pass transistor, which is connected to the MTJ by the top and bottom electrodes.

  • Motorola / Freescale

    • Sampled 4Mb MRAM in 2003, standard product by Sept, 2004.

    • Demonstrated on 90nm nodes, June, 2005

    • Believes that it can be further scaled down to 65nm

  • Cypress / NonVolatile Electronics

    • 256Kb MRAM with pin-for-pin replacements for their SRAM

  • Altis (IBM / Infineon)

    • 16Mb MRAM on 180nm CMOS technology

  • Samsung

    • Demonstrated fully integrated 64Kb MRAM with 240nm CMOS technology

  • Honeywell

    • 1Mb MRAM using 150nm CMOS technology in June, 2005

  • Other Companies: NEC / Toshiba, Sony


References
References pass transistor, which is connected to the MTJ by the top and bottom electrodes.

  • Daughton, J.; Magnetoresistive Random Access Memory; 2000.

  • Klein, L.; Single-layer PHE-based MRAM; 2005.

  • Akerman, J.; Toward a Universal Memory; Science; 308(4), 508-510; 2005.

  • Mallinson, John; Magneto-Resistive and Spin Valve Heads; Academic Press; 2002.

  • Hirota E., Sakakima H., Inomata K,; Giant Magneto-Resistance Devices; Springer; 2002.

  • Hartmann, Uwe (editor); Magnetic Multilayers and Giant Magnetoresistance; Springer; 2000.

  • Daughton, James M, Advanced MRAM Concepts; NVE Corporation, 2001.

  • Slaughter J.M et al; Fundamentals of MRAM Technology; Journal of Superconductivity; 15(1), 19; 2002.

  • Daughton, J.M.; J. Appl. Phys; 81(8), 3758.

  • Durlam, M et al.; VLSI Symposium 2002.

  • Durlam, M., et al.; A 0.18um 4Mb Toggling MRAM; Freescale Semiconductor, Inc.; 2003.

  • Pohm, A.V. et al.; IEEE Transactions on Magnetics; 33(5), 3280.

  • Slaughter, J.M., et al.; Magnetic Tunnel Junction Materials for Electronic Applications; JOM-e, 52(6); 2000:

    http://www.tms.org/pubs/journals/JOM/0006/Slaughter/Slaughter-0006.html

  • University of Konstanz, Dept. of Physics:

    http://www.uni-konstanz.de/FuF/Physik/Leiderer/Research/Dynamics_of_Thin_Films/Laser_Annealing/laser_annealing.html

  • Science and Technology Review:

    http://www.llnl.gov/str/April01/Cerjan.html

  • Automotive DeisgnLine – Memory Marquee:

    http://www.automotivedesignline.com/news/163700597

  • Honeywell MRAM Datasheet:

    http://www.ssec.honeywell.com/aerospace/datasheets/hxnv0100_mram.pdf

  • Technology Reviews – Williams Advanced Materials:

    http://www.williams-adv.com/tools/mram-technology-review.php

  • MRAM-info:

    http://www.mram-info.com/companies.html

  • IBM Almaden Research Center – Magnetic Tunnel Junctions (MTJs):

    http://www.almaden.ibm.com/st/magnetism/ms/mtj/

  • Giant Magnetoresistance:

    http://www.stoner.leeds.ac.uk/research/gmr.htm

  • How Stuff Works – RAM:

    http://computer.howstuffworks.com/ram.htm

  • Wikipedia – Various Topics:

    http://www.wikipedia.com

  • Freescale – MRAM fact sheet::

    http://www.freescale.com/files/technology_manufacturing/doc/MRAM_FACT_SHEET.pdf

  • Hyperphysics – Hysteresis in magnetic materials:

    http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html#c1


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