silpha mjt ram - 123seminarsonly.com · - 1 - seminar report 2004 dept of electronics&commn...

- 1 - Seminar Report 2004

Dept Of Electronics&Commn Engg GEC,Thrissur

1. INTRODUCTION

Impediments to main memory performance have traditionally been due to the

divergence in processor versus memory speed and the pin bandwidth limitations of

modern packaging technologies. End-user demands are forcing drastic changes in

the applications, which use semiconductor memory products, and new applications

are emerging with even more unique requirements. Some of these demands include

a reduction in power, board space and cost, as well as increased density and

performance. Flash memory has been the answer to most such demands because of

the maturity of its technology, scalability, high density and good compatibility with

the CMOS technology. But its ability to scale with technology, because of either its

tunneling oxide thickness or cell size is limited. It is struggling to keep up with the

current demands of low voltage, high speed, high retention and high endurance

operation.

As technology moves rapidly towards compact, low power, wireless and mobile

applications, a universal memory, which possesses all the good traits of different

memories like the speed of SRAM, the density of a DRAM and most importantly, an

eternal non-volatility in data storage is sought. Non-volatility, the property by

which a memory retains its data for years even with the power is turned off, is

crucial for almost all electronic devices these days. It tells a computer how to boot

up and what to do, it tells a cell phone how to send and receive calls and store phone

numbers. Different technologies are emerging to meet these demands, some of

which are ferroelectrics memory, magnetoresistive memory, Ovonic Unified

Memory (OUM), Programmable Metallization Cell memory (PMCm) etc. Different

as these technologies are, they share three main advantages over flash. First, they

can write data in a few tens of nanoseconds, like the DRAMs in a computer’s main

memory. Flash, on the other hand, takes at least a microsecond. Second, the new

memories can withstand re-writing for years whereas flash begins to lose data after



fewer than a million write cycles and thirdly, the newer memories consume far less

power than flash during its operation.

Magneto resistive Random Access Memory (MRAM), is an emerging memory

technology that exploits a property of some atoms, ferromagnetism. Atoms in a

ferromagnetic material act like tiny magnets and respond to an external magnetic

field by trying to align themselves in its direction. As in ferroelectric materials, they

arrange themselves into domains in which all the atomic magnets point in the same

direction, creating a larger magnet. Apply an external magnetic field and all of the

domains line up to point in its direction; remove the field and they remain locked in

that direction. If a field is next applied in the opposite direction, the domains flip

over. This property—that the magnetic domains of ferromagnetic materials change

direction only when they are influenced by a magnetic field—makes them ideal

memory devices. Thus an MRAM stores information using the magnetic polarity of

a thin ferromagnetic layer. This information is read by measuring the current across

an MRAM cell, determined by the rate of electron quantum tunneling, which is in

turn affected by the magnetic polarity of the cell.

2. A BRIEF HISTORY

Historically, volatile memories such as DRAM (Dynamic Random Access

Memory) and SRAM (Static Random Access Memory) were the most important

types of memory in terms of market size, despite the disadvantage of their non-

volatility. This was due to their rapid writing capability, which is mandatory for

their use as "working" memory. In the case of DRAM, the small size of the memory

cell allows very high storage capacities to be achieved: in 2003, 1-Gbit DRAMs are

commercially available, each chip with a capacity of over one billion data bits of

storage. The absence of refresh circuitry and its high reading speed are advantages

for the SRAM. Both DRAM and SRAM are essential components of PCs.

Flash memory belongs to the class of semiconductor memories called non-

volatile memories, of which it is the most dynamic driving force. These memories



are based on the EEPROM technology. The Flash memory is similar to the EPROM

cell in structure except with a high quality thinner gate oxide layer. Flash memories

are useful in applications which require more memory capacity than the EEPROM

can provide and also for applications which do not need fast and frequent

programming like memory cards for digital cameras and MP3 players. The

pressures on Flash manufacturers are twofold: to increase performance (in terms of

speed, density and power consumption) and to decrease costs.

Since 1971, anisotropic magnetoresistive (AMR) Ni-Fe alloy thin films have

been explored for use in magnetic field sensing. These types of ferromagnetic thin

films change resistance depending on the relative direction between film

magnetization and in-plane current direction. Its field sensitivity is much larger

than that obtained through coil winding. Unlike inductive magnetic field sensors,

the AMR sensor is speed independent. Devices using this type of material include

read heads in high-density hard disk drives, magnetic field sensors for a variety of

applications, and magnetoresistive random access memory (MRAM).

In 1988, a new type of magnetoresistive material, termed giant magneto résistance

(GMR) material, was discovered. The material is made of at least two magnetic

layers separated by a conducting interlayer. Its resistance depends on the relative

orientation between the neighboring magnetic layers. It is a maximum when the

directions are antiparallel and a minimum when they are parallel. Due to its

improved signal compared to AMR material, GMR films result in enhanced device

performance in most applications. GMR films have already been incorporated into

commercial read heads, and development for other device applications, such as

sensors and MRAM, is underway. The GMR films are most often used with current

flowing in the film plane. The discovery of the GMR effect was a boost to MRAM

technology. Not only is the signal strength larger, but the characteristics of the

physical phenomenon itself are well suited for MRAM, which uses magnetic-

moment direction as information storage and the resultant MR difference for

sensing.



The development of MRAM began approximately ten years ago in response to the

need for a durable, radiation-hard, nonvolatile RAM. In the early 1990s, high

magneto resistance (MR) ratio, (expressed as the change in resistance divided by the

minimum resistance) was discovered for magnetic tunnel junction (MTJ) material.

Tunneling MR values reported are in the 20-50% range, much higher than typical

GMR films. MTJ material is quickly finding applications in MRAM and magnetic

field sensing.

3. MRAM CELL

Figure 1 shows the different components of an MRAM cell. The cell is composed of

a diode and an MTJ stack, which actually stores the data. The diode acts as a current

rectifier and is required for reliable readout operation.

Figure 1. MRAM CELL

The MTJ has three functional layers as shown in figure 2. From top to bottom, there

is a free ferromagnetic (FM) layer, a tunnel barrier, and a pinned ferromagnetic (FM)

layer. In the top FM layer, the orientation of the free magnet (within the free FM



layer) in the absence of applied fields will be along either the positive or negative X

axis, depending on the state of the stored datum. In the bottom FM layer, material

anisotropy freezes the pinned magnet, to point in a fixed direction, by providing an

extremely high magnetic barrier relative to the range of fields required to switch the

free magnet. The alignment between the free magnet and the pinned magnet

governs the conductance of the MTJ. The free FM layer provides a means for storing

a bit of information, corresponding to a "1" or "0" state of a binary memory element.

Figure 2. The MTJ stack

To operate MTJ as a memory cell depicted as in figure 2 requires that the MTJ

provide a physical means for state storage, one for state detection, and another to

change its state. The MTJ memory cell stores state in one of two possible relative

orientations of its free magnet to its pinned magnet, either a parallel orientation or

an antiparallel orientation. Since the relative orientation regulates the read current

through the MTJ, a "read" circuit can detect the state of the memory cell by assessing



the MTJ resistance. The MTJ behaves as a variable resistor with two discrete

resistance values, Rparallel and Rantiparallel, where Rantiparallel is larger than the Rparallel.

These resistance values depend on the before mentioned relative orientation of the

free magnet to the pinned magnet. In other words, the relative orientation regulates

the "read current" flow through the MTJ. When the polarization is anti-parallel, the

electrons experience an increased resistance to tunneling through the MTJ stack.

Thus, the information stored in a selected memory cell can be read by comparing its

resistance with the resistance of a reference memory cell located along the same

word line. The resistance of the reference memory cell always remains at the

minimum level.

An applied field can switch the free layer between the two states, ie. to change

the state of the memory cell, magnetic fields generated by pulsed currents flowing

above and below the MTJ orient the free magnet to one of the two states. In an

MRAM array, orthogonal lines pass under and over the bit, carrying current that

produces the switching field. The bit is designed so that it will not switch when

current is applied to just one line, but will always switch when current is flowing

through both lines that cross at the selected bit.

The diode in this architecture must have a large on-to-off conductance ratio to

provide isolation of the sense path from the sneak paths. This isolation is achievable

using thin film diodes. Schottky barrier diodes have also been shown to be

promising candidates for current rectification in MRAM cells.

4. HOW MTJ MRAM WORKS?

In two-dimensional (2-D) arrays of MTJs, a single bit of information may be

selectively written by applying orthogonal magnetic fields directed within the

XY-plane of the MTJ. The combination of an easy axis field, directed along the length

of the MTJ (i.e., the X-axis), and a hard axis field, directed along the width of the

MTJ (i.e., the Y-axis), writes the MTJ to a new state. A switching astroid curve

delineates the boundary between switching the orientation of the free magnet and



not switching it. Figure 3 (a) shows an ideal switching astroid, which assumes single

domain switching. A model for the ideal switching astroid can be derived that

satisfies the relation Heasy ^ 2/3 + Hhard ^ 2/3 = Hk ^ 2/3, where Heasy is the easy axis

field, Hhard is the hard axis field, and Hk is the anisotropy field.

Figure. 3(a) ideal switching astroid 3(b) actual switching astroid

The switching astroid differs from the ideal. In general, actual switching

astroids of MTJs within a memory array are not identical. Fluctuations among them

may arise from differences in each MTJ’s shape, edge roughness, surface

smoothness, material composition, thickness of the free FM layer, pinning

mechanism of the pinned FM layer, and aspect ratio. It should be emphasized,

however, that all MTJs must have a certain consistency, as expressed by the

uniformity of their switching astroids, to serve as viable memory elements for an

MRAM chip.

Hhard and Heasy write the MTJ. Inside the switching astroids of Figure 3(a) and

(b), fields are small enough that they cannot change the orientation of the free

magnet. Outside the switching astroids, fields are large enough that they can reverse

the orientation of the free magnet, and hence through this means, the free magnet

can be set in either a parallel or antiparallel state, having a characteristically low

resistance Rparallel or high resistance Rantiparallel.



Referring to Figure 3(a), it has been explained that the MTJ may be written by

the combination of two orthogonal fields, which individually are too small to write

the MTJ: Heasy directed along the easy axis (X-axis) and represented by field point 1a

and Hhard directed along the hard axis (Y-axis) and represented by field point 2a. An

Heasy of the size of field point 1a cannot write the MTJ alone. Only a combination of

Heasy and Hhard, represented by field point 3a, exceeds the switching astroid

boundary and writes the MTJ to a state, its free magnet aligning with Heasy. Had

Heasy been negative rather than positive, resulting in the field combination

represented by field point 4a, the orientation of the free magnet would have been

reversed.

It is important to understand Hhard does not define the orientation of the free

magnet of an MTJ; instead, it only helps to destabilize the free magnet so that a

lower Heasy can be applied to switch the free magnet. This property is exploited in

two-dimensional (2-D) memory arrays where the coincident application of Heasy and

Hhard writes only the MTJ of a selected memory cell. Other memory cells that

incidentally receive one or the other field alone are not written.

The shape anisotropy (aspect ratio of the thin-film magnet) acts to confine the

free magnet such that, in the absence of a field, it will point in one of two directions;

the free magnet orients itself along the longer of the two dimensions of the thin-film

magnet which, in this example, is along the X-axis. In other words, after a switching

operation, once all external fields have been removed and the magnet has had a

chance to reach a quiescent state, the free magnet always settles in such a way that

its north and south poles align along the X-axis.

A bit of information is retrieved from the MTJ of Figure 2 by measuring its

resistance via a read current directed along the Z-axis, transverse to the XY-plane.

The logic state is extracted from the read current by determining whether the

magnitude of the read current is larger than or less than a reference current. The

reference current is halfway between a current extracted from a MTJ in a parallel

state (having Rparallel) and a current extracted from a MTJ in an antiparallel state

(having Rantiparallel).



The read current can be divided into a carrier component, henceforth referred

to as a base read current, and a signal component, henceforth referred to as a signal

current. The base current is proportional to the average of the inverse of Rparallel and

the inverse of Rantiparallel. The signal current is proportional to the difference between

the inverse of Rparallel and the inverse of Rantiparallel. The MTJ stack includes a tunnel

barrier that determines the magnitude of the base read current. The tunneling

barrier thickness and composition control the resistance of the MTJ, as measured by

the read current flowing from the free FM layer across the tunnel barrier, and to the

pinned FM layer. The base read current has an inverse exponential dependence on

the barrier thickness. The base resistance of an MTJ should be identical across a 2-D

array of such elements, with the measured resistance reflecting only a binary

difference in state among the MTJs . An MTJ's magnetoresistance provides the signal

current. The quantity of magnetoresistance is defined as the difference between

Rantiparallel and Rparallel divided by Rparallel, and it has been reported to be greater than

40%.

The quantum mechanical mechanism responsible for the magnetoresistance

of the MTJ is electron spin. The electron transport across a tunnel barrier of the MTJ

is governed by the orientation of the magnetic moment in the free layer with respect

to that in the pinned layer. "Parallel" orientation indicates, on a microscopic level,

that the spins of the conduction band electrons in each of the ferromagnetic layers

are the same, whereas "antiparallel" orientation indicates that such electron spins are

opposite. For free and pinned FM layers having the same electron spins in their

conduction bands, a high density of filled electron states furnishes a significant

number of electrons for tunneling to a high density of empty electron states,

resulting in a low tunnel barrier resistance. In contrast, for free and pinned FM

layers having opposite electron spins in their conduction bands, a lower density of

filled electron states furnishes fewer electrons for tunneling to a high density of

empty electron states, resulting in a high tunnel barrier resistance. Hence, an MTJ

having a "parallel" orientation between moments has a lower resistance than one

having an "antiparallel" orientation.



5. 1T1MTJ MRAM

One flavor of MRAM is the one-transistor-one-MTJ (1T1MTJ) memory cell,

offering very high performance. Figure 4 depicts a 1T1MTJ MRAM circuit

comprising an array of 1T1MTJ memory cells, row drivers, bit line (BL) connect

circuits, a sense amplifier, and write circuits. Each 1T1MTJ memory cell consists of

one n-type transistor, which is used to isolate selectively the read current of one

memory cell from that of another (sharing the same bit line), and one MTJ, which

provides nonvolatile coverage.

Figure 4. 1T1MTJ MRAM circuit

It should be emphasized that the bit line connects directly to each MTJ along a

column of memory cells, while the write word line (WL) passes under the MTJs

along a row of memory cells. The write word line is electrically isolated from the

memory cells.



6. Write Operation of 1T1MTJ MRAM

A selected MTJ within the array of 1T1MTJ memory cells is written by the

coincident application of orthogonal magnetic fields, a hard axis field that emanates

from a selected write word line (WL) and an easy axis field that emanates from a

selected bit line. Write currents Iword and Ibit generate circumferential magnetic fields

Hhard and Heasy in the range of 20 to 80 Orstead. The magnitude of the write current

required to switch the MTJ depends upon the switching astroid (threshold) of the

free magnet.

From elementary physics, it is known that the magnitude of the magnetic

field applied to an MTJ is directly proportional to the current flowing through the

source conductor, the bit line, or word line, but it is inversely proportional to the

radial distance from a source conductor to the MTJ itself. Therefore, the distance

from the center of the write word line to the MTJ, deltaZ, should be minimized to

maximize the magnetic coupling.

Traversing a selected write word line and a selected bit line within the

1T1MTJ MRAM Circuit of Figure 4, Iword and Ibit induce magnetic fields Hhard and

Heasy, respectively, that together write a memory cell. A word address directs a row

driver to steer Iword from a word write circuit through a selected row driver, through

the selected write word line, and finally- to the ground return line. Likewise, a bit

address directs bit line connect circuits to steer Ibit from a write circuit designated to

operate as a source, through a bit-line connect circuit, through the selected bit line,

through another bit-line connect circuit, to another write circuit designated to

operate as a sink. A datum input signal determines which write circuit operates as a

current source and which operates as a current sink. The sign of Ibit determines the

state of the datum to be stored in a selected memory cell. Inverting the data input

reverses the direction of Ibit and Heasy thereby writing a complement state (instead of

a true state) in the selected memory cell. A coincidence Of Hhard and Heasy fields

drives the free magnet of the MTJ of the selected memory cell to either a parallel or



an anti parallel orientation with respect to its pinned magnet. In Figure 3, the

composite field applied to the MTJ corresponds to field point 3a. In the process of

writing the selected cell with Hhard and Heasy fields, other memory cells are subjected

to one or the other of the two fields.

An unfortunate by-product of the coincident application of fields on the

selected memory cell is the partial stimulation of the remaining memory cells, which

are not intended to be written, by Hhard and Heasy . These half-selected cells occupy

the row of cells along the selected word line or the column of memory cells

connected to the selected bit line. Referring to Hhard and Heasy, field points 2a and 1a

of the switching astroids of Figure 3, since the half-select fields applied to the

half-selected cells reside within the switching astroid curve, they should not disturb

the state of the half-selected cells. However, additional stray magnetic fields could

combine with the half-select fields in such away that the cumulative fields could

exceed the astroid curves causing the half-selected cells to switch inadvertently. It is

therefore necessary for the designer to consider all significant factors that impact the

absolute margins between selected and half-selected cells including stray field

coupling, misalignment of the write WLs and Us with respect to the MTJs, non-

idealities of the write circuit, and the non-idealities of MTJ itself.

For example, the impact of stray field coupling can be quantified in terms of

Hhard and Heasy fields applied to the half selected cells and the selected cell. Hhard and

Heasy fields extend beyond the dimensions of the selected MRAM cell, spilling over

into neighboring, unselected cells. According to the equation that expresses the field

strength emanating from an ideal conductor, the strength of Hhard or Heasy decays as

the inverse of the radial distance outward from the center of the corresponding

source conductor, either the write word line or write bit line, to the free FM layer of

the MTJ. In addition , only the in-plane field component of the total magnetic field

switches the MTJ. It can be deduced from these relationships that a fraction of the

in-plane Hhard field, applied to the selected row of memory cells, couples to memory

cells in neighboring rows according to

Hcouple ~ Hhard / [(I + deltaY / deltaZ)] 2 (1)



In this equation, Hcouple is the fraction of the Hhard field that is coupled to the

nearest neighbor cell, deltaY is the horizontal spacing between MTJs along the Y

axis, and deltaZ is the vertical distance between the free FM layer of the MTJs and

the center of the write word line conductor. The equation for Hcouple would be more

complex if misalignment between the MTJ and the write word line was considered,

or if the finite width and height of the write word line were included, but,

nonetheless, it can be revised to include such non-idealities. For some memory cell

geometries, the Hcouple can be even higher than 10% of the Hhard Thus, it is should be

emphasized, once again, that only precise control of the magnetic circuit and the

MTJ will yield a memory array with enough write-margin to write the selected

memory cell only and not disturb the state of neighboring memory cells or other

half-selected memory cells.

7. Read Operation of 1T1MTJ MRAM

During a read operation, a selected read word line enables n-type transistors

along a row of memory cells in the 1T1MTJ MRAM circuit of Figure 4.

Consequently, only the MTJ of each selected memory cell is connected from a bit line

to ground. A read current (not shown) originating in a sense amplifier traverses a

path including a bit line connect circuit (to read port) and the selected 1T1MTJ cell

and ending at ground. The n-type transistor directs the read current supplied by the

sense amplifier to flow exclusively through the selected MTJ, thereby shielding it

from the influence of other MTJs attached to the same bit line.

The sense amplifier detects the logic state stored within a selected memory

cell. Its positive terminal (or negative terminal) connects through a bit line connect

circuit to the selected memory cell, via the bit line, and its negative terminal (or

positive terminal) connects through another bit line connect circuit to a reference

cell, via a reference line. Referred to as a voltage-clamped-current sense amplifier,

this sense amplifier ideally forces its positive and negative terminals to the same

voltage by supplying two different currents, one having a variable value and the



other having a fixed value. A read current and a reference current flow from the

sense amplifier to the memory cell and the reference cell, respectively. These

currents are compared within the sense amplifier to determine the logic state of the

memory cell, having been modulated by the resistances of the memory cell and of

the reference cell. If the read current is greater than the reference current, then the

sense amplifier would detect a "1" state stored within the memory cell, for example

(the "1" and "0" logic state assignment is arbitrary for the sense amplifier as long as it

is consistently defined with respect to the write circuit); whereas, if the read current

is less than the reference current of the reference cell, then the same sense amplifier

would detect a "0" state within the memory cell.

Although it is possible to design a reference cell that provides a current

halfway between the read current of a memory cell storing a "I" state and the read

current of one storing a "0" state.In large-scale manufacturing, the reference cell is

likely to produce inconsistent current levels, due to variations in its shape or size

with respect to a memory cell's shape or size. It is therefore advisable to develop

accurate and consistent reference currents through a network of preprogrammed

memory cells. The current averaging reference circuit of Figure 5 accomplishes such

a function.



Figure 5. Current averaging reference circuit

For simplicity, sense amplifiers 1 and 2 are shown connected to selected BLs

1 and 2 and reference lines I and 2, the exact connections to the selected BLs and

reference lines having been formed within the bit line connect circuits (to read port)

of Figure 4 in response to an applied bit address. Similarly, in response to an applied

word address (shown in Figure 4), an enabled read word line closes all switches of

Figure 5 (the n-type transistors of Figure 4) along the selected row, thereby

connecting the variable resistors of Figure 5 (representing the MTJs of Figure 4) to

ground. Selected 1T1MTJ memory cells have closed switches. Unselected 1T1MTJ

memory cells have open ones. The reference cells in the circuit are formed out of

preprogrammed memory cells in a "1" state or a "0" state having resistances Rparallel

or Rantiparallel, respectively.

A voltage bias, is applied across both the selected memory cells and the

reference cells. A reference current for each sense amplifier is established by

averaging the "1" and "0" currents of reference cells. The connection between



reference line 1 and reference line 2 combines "1" and "0" currents from two

reference cells to form the 2X reference current that then divides into two equal

reference currents, one for sense amplifier 1 and the other for sense amplifier 2. This

reference scheme insures the reference current tracks halfway between the "0" and

"1" current of the memory cells since the same voltage Vbias is applied across both

the MTJs of the selected memory cells and the MTJs of the reference cells. Having a

common voltage Vbias applied across all MTJs is important because the

magnetoresistance of an MTJ has a nonlinear dependence on voltage.

8. CROSS-POINT CELL (XPC) MRAM

Figure 6 depicts an XPC MRAM circuit proposed as a dense alternative to the

1T1MTJ MRAM circuit of Figure 4.

Figure 6. XPC MRAM circuit



In this alternative, MTJs are used exclusively to form the memory cells and

are sandwiched between, and electrically connected to, orthogonal WLs and BLs as

depicted in Figure 7. Advantageously, the XPC memory cell requires one MTJ only,

as opposed to a MTJ plus a switching device in the IT1MTJ memory cell. Moreover,

a single WL in XPC MRAM serves both write and read functions for a row of

memory cells. In contrast, the 1T1MTJ MRAM circuit of Figure 4 requires two WLs

for each row of memory cells, a dedicated read WL and a dedicated write WL.

Unfortunately, though, for XPC MRAM, duplicate WL, circuits are required to

control the hybrid read-write functionality required by the single WL approach.

Figure 7. Array of MTJs for XPC MRAM

With a simpler cell and one WL, the XPC memory cell offers a minimum cell

area of 4F2. Both WLs, laid out in metal-2 (M2), and BLs, laid out in metal - 3 (M3),

have a metal width and spacing off, resulting in an MTJ area to 1F2. F refers to the

minimum feature size, where currently F is less than 0.18µm for leading-edge

technologies. Actually, the memory cells size and the MTJ area are larger than 4F2

and 1F2,respectively, since the MTJ needs to be elongated along the X-axis in order

to fix the easy axis orientation of the free magnet along the X-axis. Without shape,

anisotropy, the magnetic moment in the free FM layer would orient itself in an

arbitrary direction, destroying the intended binary storage capability.

Only back-end metal levels (e.g., M2 and M3) are needed for accessing the MTJ,

thereby leaving the silicon area underneath the storage layer unoccupied. This active

area can be used for peripheral circuits, such as decoders or logic blocks, in order to

reduce overall chip size. By modifying only the back-end layers, an XPC MRAM can



be added above existing circuits, making the XPC MRAM a promising candidate for

embedded and high-density applications. Moreover, 3-D stacking of MTJs is

-possible, leading to cell areas close to 4F2/n where n refers to the number of stacked

storage layers. A 3-D XPC array uses two stacked MTJs, which leads to a cell size

close to 4/2F2. While Flash memories store multiple bits in the device threshold of

one transistor, XPC MRAM stacks multiple bits one on top of the other.

In Figure 6, a schematic block diagram of the XPC MRAM is shown. The

write operation of the XPC MRAM employs the same coincident field selection

mechanism (Heasy and Hhard) as that used in 1T1MTJ MRAM. However, to generate

the same Hhard 011 the free magnet of the MTJ, the XPC MRAM requires less write

current 1word, than the 1T1MTJ MRAM, due to improved magnetic coupling

gained from the reduction of the vertical distance (along the Z-axis) of the XPC

memory cell (as depicted in Figure 7) in reference to the vertical distance, deltaZ, of

the 1T1MTJ memory cell, the vertical distance being measured from the center of the

WL to the free ferromagnetic layer. In addition to the WL height and the MTJ height

of the XPC memory cell, the 1T1MTJ memory cell of Figure 5 has the intervening

heights of insulating layer and MX layer totally.

XPC writing is illustrated in Figure 8. Currents are forced through WLi and

BLk to write the selected cell S at the cross section of WLi and BLk Since a cross-point

cell array is a resistive network, the voltage drop along the selected lines WLi and

BLk caused by the wire resistance and the MTJ resistance induces parasitic currents

that flow into unselected WLs and BLs tied to the equipotential voltage, Veq, supply.



Figure 8. XPC writing

The variation of the programming current along the WL or BL depends on

the resistance of the MTJs, the number of MTJs connected to the WL or BL, and the

resistance of the WL or BL. In order to reduce the variation of the programming

current along WLi/BLk caused by the parasitic currents, the resistance of the MTJ

cells has to be significantly higher than in a 1T1MTJ design. An increased MTJ

resistance, however, leads to decreased read signal levels, and the XPC MRAM,

therefore, requires more sophisticated sensing circuitry than the 1T1MTJ MRAM

required.

The read operation of the XPC MRAM differs from the read operation of the

1T1MTJ MRAM in that the unselected WLs and unselected BLs are held at the

equipotential voltage Veq by a keeper device within the peripheral circuits while the

selected word line is grounded. This behavior is explained with reference to Figure

9.



Figure 9.XPC Reading

Ideally, the sense amplifier forces Veq across the selected memory cell (or

cells) and zero voltage drop across the unselected memory cells; the read current

would then depend only on the state of the selected memory cell (or cells), labeled

Rc. Unfortunately, mismatches in the threshold of the transistors clamping the BLs,

WLs, and sense amplifier to Veq cause small voltage drops (Vos) to develop across

unselected memory cells, reducing the signal-to-noise ratio of the XPC MRAM. The

auto-zero sense amplifier of Figure 9, however, can correct such mismatches with its



Vos compensation scheme (An auto-zero sense amplifier removes offset in an

amplifier by first measuring offset voltage at the input of the amplifier, next storing

the measured offset on a capacitor, and finally summing the measured offset with

the input to cancel the offset in the amplifier).

9. ADVANTAGES OF MRAM

MRAM cells are non-volatile, and they can be both faster, and potentially as

dense, as DRAM cells. They can be implemented in wiring layers above an active

silicon substrate as part of a single chip. Multiple MRAM layers can thus be placed

on top of a single die, permitting highly integrated capacities, MRAM has the

potential to be fabricated on top of a conventional microprocessor, thus providing

very high bandwidth. The access time and cell size of MRAM memory has been

shown to be comparable to DRAM memory.

As the data stored in an MRAM cell are non-volatile, MRAMs do not

consume any static power. Also, MRAM cells do not have to be refreshed

periodically like DRAM cells. Thus, MRAM memory has attributes which make it

competitive with semiconductor memory.

The memory technology comparison table 1 compares the attributes of MTJ

MRAM (both IT1MTJ and XPC MRAM) to the attributes of other RAMs, specifically

SRAM, DRAM, NAND Flash, and NOR Flash. In the nonvolatile category, MTJ

MRAM offers practically infinite high write endurance, expected to exceed 1015

cycles, and high-speed write access, between 10 to 40 ns. Moreover, IT1MTJ MRAM

offers a smaller cell size than SRAM. This would be particularly attractive for the

wireless device arena where data endurance and nonvolatility have become the

"mantra." In comparison to 171MTJ MRAM, XPC MRAM offers higher density at the

expense of READ random access time.



Table 1.Comparison of memory technology

SRA

M

DRAM NAND

FLASH

NOR

FLASH

1T1MTJ

MRAM

XPC

MRAM

Cell size

(in F2)

100 8 5 6 >8 >4

Supply

(V)

2.5 2.5 1.8 3.3 1.8 1.8

Retention

power

(mW)

375 10 0 0 0 0

Retention

time

∞ 64ms 10yrs 10yrs 10yrs 10yrs

Random

access

read

2-100

ns

60ns 10µs 90ns 10-50ns 50ns-

1µs

Random

access

write

2-100

ns

60ns 100µs 100µs 10-40ns 20-40ns



10. DRAWBACKS Unsurprisingly, MRAM devices have several potential drawbacks. They

require high power to write, and layers of MRAM devices may interfere with heat

dissipation. Furthermore, while MRAM devices have been prototyped, the latency

and density of production MRAM cells in contemporary conventional technologies

remains unknown. To justify the investment needed to make MRAMs commercially

competitive will require evidence of significant advantages over conventional

technologies.

11. CHALLENGES

One of the challenges involved in the integration of MRAM technology is

temperature compatibility with the CMOS process. Several standard CMOS process

steps occur at or above 400°C. As shown in figure 11, the MR of typical MTJ material

begins to degrade at temperatures above 300°C and drops sharply by 400°C.

Figure 10.



Thus, for a working memory either the MTJ material must be improved to

withstand these standard process temperatures, or low-temperature processes must

be developed for MRAM technology. For our demonstration circuits, special low-

temperature processes were used to prevent the MTJ material from being exposed to

higher temperatures during MRAM processing. Improvements in the thermal

endurance that would make the materials compatible with standard processes

would enhance the manufacturability of the technology.

Obtaining very uniform RA over large wafers is another challenge.

Techniques that have been explored include forming the aluminum-oxide tunnel

barrier with air; reactive sputtering; plasma oxidation with plasma source; plasma

oxidation with power introduced from the target side; and plasma oxidation with

power introduced from the substrate side. The results show that all techniques can

be made to work. Plasma oxidation is favored due to its simplicity and

manufacturing compatibility. It was also discovered that different oxidation

methods used in this study caused little difference in MTJ resistance uniformity. The

latter is mainly determined by the aluminum metal thickness uniformity. Modeling

based on Simmons' theory supports the experimental finding. This illustrates that

the key to better MTJ RA uniformity is to improve the aluminium metal layer

thickness uniformity.

A final challenge is producing MTJ material with very low RA. As bit

sizes are reduced, MRAM may require material with lower RA. In addition, use in

hard-disk read heads would require a much lower resistance for the first generation

of product. Obtaining a thinner tunnel barrier without losing MR is one of the key

factors to achieving low RA.



12. CONCLUSION

There is no doubt that there is a need for a new memory technology for a

successful market penetration and a better end user product. Lower costs, lower

power consumption, non-volatile techniques, and the new technology should be

easy to integrate into existing CMOS technology. Comparing these requirements for

future memories with current memory devices, we see that each of them has certain

limitations: DRAMs are difficult to integrate, SRAMs are expensive and FLASH

devices are too slow and have a limited number of write/erase cycles. EEPROMs

show high power requirements and a poor flexibility. None of them combines

features like: The ability to retain stored charge for long periods with zero applied or

refreshed power, high speed of data writes, low power consumption, large number

of write cycles. Which in turn are met by MTJ MRAM.

In the race to commercialize MTJ MRAM, process engineers and physicists

are battling to increase MTJ bit yield, to improve the intrinsic write-margin of the

MTJ, and to incorporate magnetic materials (nickel and iron) into the back-end

process. Circuit designers are developing clever approaches to increase memory

density, to minimize power consumption, and to reduce read access times.

Considering the current pace of development, MTJ MRAM technology could

become a mainstream memory technology of tomorrow.



REFERENCES

• IEEE Circuits and Devices

• domino.research.ibm.com

• www.public.asu.edu

• www.cordis.lu/esprit/src/28229.htm

• www.tcd.ie/Physics/Magnetism/

Conferences/tfdom3/parkin.pdf

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