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2B1750 Smart Electronic Materials

Non-Volatile Random Access Memory Technologies(MRAM, FeRAM, PRAM)

Muhammad Muneeb

[email protected]

Imran [email protected]

Aftab Nazir [email protected]



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ABSTRACT

Today’s computer and information technology is strongly connected to the advancements in datastorage technology. Presently the information being processed is stored in Dynamic RandomAccess Memory (DRAM).This RAM is quite fast but a big disadvantage is its volatile nature.While today’s high performance computing requires non-volatility of memory. Researchers aretrying to find the new technologies and material to build a non-volatile RAM which is fast, dense,low at power comsumption and economically favorable. A number of options are being

considered but here in this report we will focus on three most promising options namelyMagnetoresistive Random Access Memory (MRAM), Ferroelectric Random Access Memory(FeRAM) and Phase Change Random Access Memory (PRAM). We will be discussing thetechnology and physics involved in implementation of each of them and the current major challenges being faced for their commercial realization. Towards the end we will compare all of them and will propose the future universal RAM.

Keywords: Magnetoresistance, GMR, TMR, MTJ, Ferroelectricity, Pervoskite, PZT,Chalcogenide glass, GST



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CONTENTS

1.1 Magnetoresistive Random Access Memory (MRAM)1.2 Effects and Materials: MRAM

1.2.1 Giant Magnetoresistance (GMR)1.2.2 Tunnel Magnetoresistance (TMR)

1.3 Implementation of MRAM1.4 Major Challenges for MRAM2.1 Ferroelectric Random Access Memory2.2 Selected Ferroelectric Material for FeRAM2.3 Effects and Properties of Ferroelectric materials

2.3.1 Ferroelectric Domains and Hysteresis

2.3.2 Curie point and Phase Transition2.4 Implementation of FeRAM2.5 Challenges for FeRAM

2.5.1 Polarization Fatigue2.5.2 Retention Loss2.5.3 Imprint

3.1 Phase Change Random Access Memory (PRAM)3.2 Effects and Materials: PRAM

3.2.1 Potential Materials and Effects3.2.2 Crystal Structure of GST (Ge2Sb2Te5 )3.2.3 Why (Ge2Sb2Te5)?

3.3 Implementation of PRAM

3.3.1 Voltage, current waveforms and temperature for the set and reset3.3.2 Current Status of PRAM3.4 Major Challenges for PRAM:-4 Conclusion5 References



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1.1 Magnetoresistive Random Access Memory (MRAM)

Fast, non-volatile and random accessed magnetic storage of data was realized in 1960s for veryearly computers in the shape of magnetic core memory. As shown in figure1.1 the memory cellsconsist of wired threaded tiny ferrite toroids. With two states of remanent polarization binary “1”

and 0” are stored. Now with the development of integrated electronic circuits the bulky ferritetoroids has been replaced by thin magnetic layer elements. This memory has the same writing

principle while for reading more sensitive magnetoresistive effects are being employed. That’swhy now it is known as Magnetoresistive Random Access Memory (MRAM).

Figure 1.1 Magnetic core random access memory

1.2 Effects and Materials: MRAM

In ferromagnetic materials the motion of an electron may depend on the spin orientation withrespect to the local magnetization [1.1]. The effects which arise from this fact are stronger if thespin is conserved during the process of interest. The length scale for conservation of spin, spindiffusion length, varies from few nanometers to several tens of nanometers. For this reason thinfilm multilayers and their interfaces plays an important role for integrated circuit applications. Inthe subsequent section we will discuss some interesting and useful effects which can be used for implementation of MRAM

1.2.1 Giant Magnetoresistance (GMR)In layered magnetic structures the resistivity depends on the relative alignment of the adjacentferromagnetic layers. Early experiments were performed on Fe/Cr multilayers. As shown in

figure 1.2 at zero field the adjacent ferromagnetic Fe layers align antiparallel due toantiferromagnetic interlayer exchange coupling across the Cr spacer [1.4]. When sufficientexternal field is applied the samples saturates and a parallel alignment occurs. This change fromantiparallel to parallel state give rise to drastic change in resistivity of sample. This effect wasmuch bigger than the change of resistivity due to the difference in angle between current andmagnetization (anisotropic magnetoresistance: AMR) so this new effect was termed as giantmagnetoresistance (GMR).



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Figure 1.2 First observation of GMR effect for Fe/Cr/Fe trilayers [1.4]

If one of the layer is magnetically pinned and the other layer’s magnetization can rotate whileapplying external, this is called as spin valve. The layer whose magnetization is fixed is calledhard layer and the one whose magnetization can rotate is called soft layer. The GMR effect isreported in literature as % ratio of change in resistance in parallel alignment to antiparallel case.If Rp is the parallel case resistance and Rap is antiparallel case resistance then

GMR % = ? R/Rp = (Rap- Rp) / Rp (1)

The GMR effect is usually observed in two different geometries of CIP(current in plane) and CPP(current perpendicular plane). However in CPP geometry the effect is relatively stronger. Atmicroscopic level The GMR effect can be explained by Motto’s two current model in which he

assumes two different current channels for spin up and spin down electrons. The scattering process especially spin dependent reflectivity is responsible for electrical resistance.[1.4] Onlythe states near the fermi energy can contribute to conductivity because they can reach the emptystates above fermi energy after some scattering event. In figure 1.3 simplistic picture of scatteringis shown where no scattering occurs for majority carrier (spin up electron in up magnetization)where minority electron suffer scattering from both spacer layer interface and adjacentferromagnetic layer. As the resistance for both spin up and spin down electron current channelsare in parallel. Thus foe parallel magnetization case Rp is almost short circuited and for antiparallel magnetization case Rap will have some finite value. For above discussion weassumed that spin direction is conserved in spacer layer which will be only true for a very thinlayer compared to spin diffusion length.



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Figure 1.3 Simplistic picture of spindependent scattering for explanation of GMR effect[1.4]

If we can increase the trilayers to multilayers the GMR effect can be increased due to theincreased probability of scattering in multilayers. The GMR effect representative values asdefined by equation (1) have been reported in literature which is being compiled in followingtable. Number in brackets indicate the layer thickness in A?[1.4]



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1.2.2 Tunnel Magnetoresistance (TMR)

Tunnel magnetoresistance effect is observed in a configuration when two thin film ferromagnetic

electrodes are separated by an insulating or semiconducting barrier. Now if a small voltage isapplied across these electrodes small quantum mechanical tunnel current can flow across the barrier. From this it is evident that TMR effect can only be seen in CPP (current plane perpendicular) geometry. The tunnelcurrent is related to the overlap of exponentially decayingwave functions within the barrier thus, the current will decrease with the increasing thickness of

barrier. Typical thickness of barrier is of order of 1 nm. In most cases AlOx is used as insulating barrier. The tunneling resistance depends on the relative magnetization on both sides of the barrier. TMR effect is defined in the same way as GMR. Rp is for parallel magnetization on bothsides of barrier and Rap is for antiparallel case.

TMR % = ?R/Rp = (Rap- Rp) / Rp (2)

Values of TMR in the range of 40% to 50 % have been reported in literatureTMR effect can be understood by spin polarized tunneling. If spin is conserved during tunneling(which is only possible is barrier thickness is of order of spin diffusion length) an initially spin upelectron state can only tunnel to a spin up final state and vice versa. The TMR effect arises fromthe difference in spin up and spins down tunneling electrons.[1.4] Thus in terms of spin

polarization TMR can be defined mathematically as

TMR % = ?R/Rp = (Rap- Rp) / Rp = 2PLPR / (1- PLPR ) (3)

Where PL and PR are the spin polarization in left and right electrodes

1.3 Implementation of MRAM

Both magnetoresistive effects discussed in the previous section can be used to implementMRAM. However, TMR is more attractive because it has higher value of magnetoresistance andhas been shown as high as 50% for NiFe, NiFeCo and CoFe ferromagnetic electrodes. Additionalferromagnetic and antiferromagnetic layers can be added in order to fix the magnetization for oneferromagnetic layer to one direction and to provide stability to magnetic material. The Al2O3 tunnel barrier can be engineered by optimizing Al thickness and oxidation time. Schematic of such a MRAM cell based on MTJ (magnetic tunnel junction) is shown in Figure 1.3 andresistance versus field response of such a material is shown in figure 1.4



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Figure 1.3 Schematic of a MTJ (Magnetoresistive tunnel junction)[1.7]

Figure 1.4 Magnetoresistance versus field response of MTJ[1.7]

The top layer is a free layer which is typically soft magnetic alloy such as NiFe that reverseseasily when a small external field is applied. The bottom layers give an effect of fixed layer due

to interlayer exchange coupling between ferromagnetic and spacer layer of syntheticantiferromagnetic . When a negative field is applied the free layer becomes antiparallel to thefixed CoFe layer across the tunneling barrier and the resistance is high. Conversely when positivefield is applied the free layer aligns parallel to that of fixed layer and thus resistance is low. Thehyteresis loop suggests that when the field is removed the material still remains in its state unlessa magnetic field in reverse polarity is applied to change the bit status. Thus the proposed memoryis a non-volatile RAM.Actual reading and writing mechanisms can be understood by considering figure 1.5. In writemode the transistor is kept in OFF state and current is passed through lower conductor which

produces a field the hard axis direction and thus tilts the free layer magnetization. Only the bit inwhich current is applied in both hard and easy axis will be written other bits will remain half select. For read mode the transistor is turned ON and a small sense current passed from MTJ to

the ground.



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Figure 1.5 write mode (a) and read mode (b) of a typical MRAM cell[1.4]

1.4 Major Challenges for MRAMMajor challenges have to be solved for MRAM to really compete with other non-volatile RAMtechnologies. First is the thermally activated reversal of the free layer which leads to increase inerror rate. A certain level of energy barrier has to be maintained for error free non-volatilityfunction and this energy barrier to K BT ratio must be fairly large (>50). Second challenge isswitching induced demagnetization of the fixed layer which leads to loss of read-out signal which

is analog of wear out in other memory technologies.[1.4]Third challenge is to integrate TMR with current CMOS processing technology.

2.1 Ferroelectric Random Access Memory

Ferroelectric random access memories (FeRAMs) are the next generation future memories due to

high speed, low cost, low power, nonvolatality and good compatibility with the existingintegrated circuit(IC) technology. It offers higher endurance (the no of read and write cycles amemory can undergo before losing the ability to store data) to multiple read and write operations.

2.2 Selected Ferroelectric Material for FeRAM

A perovskite-type structure (ABO3) is used as Ferroelectric material. Typically speaking



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PZT (Pb {ZrTi}O3) .By applying or removing the external e-field the internal atom Zr/Ti can be displaced up or down into multiple stable state positions, which creates a permanent electrical polarization of the material, resulting the non-volatile property of the material. As a result the power consumption is very low for the data storage. Lead Zirconate Pb (Zr x,Ti1-x)O3 ( PZT) andstrontium bismuth tentate SrBiTa2O9 are the most promising candidates for the future FeRAMS.

[2.5]

2.3 Effects and Properties of Ferroelectric materials

The property of certain dielectrics, that exhibits spontaneous electric polarization i-e (theseparation of the centre of positive and negative electric charge, leading one side of the crystal

positive and other negative) that can be reversed in the direction by applying certain suitableelectric field. It is the reversible spontaneous alignment of electric dipoles by their mutualinteraction.[2.9]A Ferroelectric effect is characterized by the remanent polarization which occurs when an e-fieldhas been applied. Such Ferroelectric materials allows single ion to change its physical location.

For instance in a simple model of a Ferroelectric material like PZT or SBT the centre atom either Zirconium or Titanium will move into one of the two stable states upon an external appliedelectric field. After the removal of the external electric field, the atom remains polarized in either state. That is actually which leads to the non-volatility of memory in Ferroelectric. Reversal of the polarization state of the center atom can be done by applying the e-filed causes the changefrom a logic `0`to `1`or `1`to `0`.[2.2]



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Figure 2.1 Two stable states in a Ferroelectric material (PZT) .The orientation of the spontaneous polarization is reversed by applying a proper e-field.[2.2]

The central atom Ti/Zr in the above unit cell of PZT will be displaced from its previous stable position by applying a suitable e-field and thus changes the polarization state of the unit cell.

A ferroelectric material must have a spontaneous dipole moment which can switch in an electricfield. This is found when two particle of charge q are separated by a small distance r, called theelectric dipole

As the ability of a crystal to exhibit the spontaneous polarization is related to its symmetry. The polar properties are not possible in Centrosymmetric structures because any dipole momentgenerated in one direction would be forced by symmetry to be zero. Thus in Ferroelectricmaterial, there is a permanent dipole moment which is the sum of dipole moments in each unitcell. That is the Ferroelectrics must be non-centrosymmetric. However this is not only therequirement. There must be spontaneous local dipole moment (which typically leads to amacroscopic polarization, but not necessarily if there are domains that cancel completely).Thismeans that the central atom must be in a non equilibrium position. For instance consider an atom

in a tetra interstice,

2.3.1 Ferroelectric Domains and Hysteresis

Ferroelectrics form the domain structures. A domain is a region where there is the uniformdirection for the spontaneous polarization. In a domain the electric dipoles are aligned in the samedirection. The boundaries between the domains are referred to as domain walls.The domain boundaries are usually described according to the angle between the domains thatthey separate. The most common found are 90° and 180° boundaries. While growing a singleFerroelectric crystal, it has a number of Ferroelectric domains form which a single domain can beseparated by the motion of the domain wall by applying an appropriate electric field. The

polarization reversal in the domain, called Domain Switching, can happen by applying a strongelectric field. That is actually the main feature that differentiates between the pyroelectric andferroelectric materials. [2.7, 2.10] We can observe the polarization reversal by measuring the Hysteresis as shown in the figure-

below .By increasing electric field strength the domains start to align in the positive directioncauses an increase in the polarization (OB).At very high value of electric field the polarizationreaches to its saturation (Psat).The polarization does not fall to zero as the electric field is switchedoff. At zero external fields some domains will remain align in the positive direction. Thereforethe crystal will show a remanent polarization Pr. For the crystal to be completely depolarized afield of magnitude OF is applied in the negative direction. In order to reduce the polarization tozero, a negative electric field called the coercive field Ec is necessary. If the negative field isfurther increased, the hysteresis loop is followed in the reverse direction. The extrapolation of the

line CE towards zero electric field gives the saturation polarization Ps (OE) at E=0.



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Figure 2.2 A Polarization vs. Electric Field (P-E) hysteresis loop for a typical ferroelectriccrystal.[2.4]

2.3.2 Curie point and Phase Transitions

The Curie point or the Curie temperature, Tc,is the temperature above which the Ferroelectricmaterial loses its spontaneous polarization.Ferroelectric materials have this Curie point (Tic).At T>Tc the crystal does not behave asFerroelectric, where as for T<Tc it behaves as Ferroelectric. As the temperature is decreased fromthe Curie point, a Ferroelectric crystal exhibits a phase transition from a non-Ferroelectric phaseto a Ferroelectric phase. In case of large number of Ferroelectric phases the crystal undergoesfrom one Ferroelectric phase to another at a certain temperature called the transition temperature

[2.10].In the figure below the variation of the relative permittivity with temperature for a certain crystal(BaTiO3) as it is cooled from its cubic phase to the Ferroelectric tetragonal, orthorhombic andrhombohedra phases. Near the transition temperature the elastic, dielectric, optical and thermalconstants execute irregular behavior due to distortion in the crystal as the crystal undergoes phasestructure change. Above the Curie point (T>Tc) the temperature dependence of the dielectricconstant in Ferroelectric crystal is governed by the Curie-Weiss law,[2.4]

e = e 0 + C/(T-To) (4)Where e is the permittivity of the material, e 0 is the permittivity of vacuum; C is the Curieconstant and to is the Curie temperature. The Curie temperature to be different from the Curie

point Tc. To is a formula constant obtained by extrapolation, while Tc is the actual temperature

where the crystal structure changes. For first order transitions To < Tc while for second order phase transitions To = Tc.



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Figure 2.3 Variation of dielectric constants (a and c axis) with temperature for BaTiO3

[2.4]

2.4 Implementation of FeRAM

In a Ferroelectric Ram a ferroelectric film is used as capacitor to hold the data as it has thecharacteristic of remanant polarization which is reversed as by applying the electric field givingrise to the P-E hysteresis loop. In order to reduce the operational voltages the capacitors of thickness of the order of sub micron can be prepared by using the thin film technology.FeRAMuses the P-E characteristic to store the data in a non-volatile state and allows the data to berewritten fast and frequently.In ferroelectric capacitor,the dielectric layer is replaced with a thinferroelectric flux,typically made of lead Zirconate titanat

Figure 2.4 Ferroelectric capacitors layers (two electrodes and thin film of ferroelctric material)on top of the conventional C-MOS process.[2.1]

It uses the voltage pulses to read and write the digital information. If the electrical field of theapplied pulse is in the same direction as that of remanant polarization, there will be no switching.



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The dielectric response of the ferroelectric material causes the change of polarization ?Pns is dueto the dielectric response. If the initial polarization is in opposite state due to the field, the

polarization reverses causes an increased switching polarization change ?Ps. The different statesof remanant polarization (+P r and -Pr ) cause the different transient current behavior of theferroelectric capacitor to an applied voltage pulse. By integrating the current, switched ?Qns andthe non-switched charge ?Qns can be determined. The difference in charge ?Q=A?P enables to

distinguish between the two logic states.

Figure 2.5 Non-switching and switching of the polarization of a ferroelectric capacitor andcurrent response [2.10]

2.5 Challenges for FeRAM

As non-volatility is the specific function of the FeRAMs, so need to develop the methods for the reliability test. three different failure mechanisms are observed effecting the operation of theFeRAMs. For the realization of the commercially available FeRAMs, some technical issues haveto be addressed. Under some conditions the ferroelectric materials may degrade, causing theunreliability.

2.5.1 Polarization Fatigue

Fatigue is the decrease of polarization charge after writing and reading.Due to the continuouscycling the positive and the negative saturation in the hysteresis loop, which shows thecontinuous read and write operation of the same cell, the p-v hysteresis loop becomes flatter and

the remnant polarization is decreased.



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Figure 2.6 Pulse schemes for measuring fatigue behavior and fatigue effect of hysteresis of ferroelectric capacitor[2.10]

The difference between the switching and the non-switching charge becomes smaller causing thefailure of the device if this difference is too small to be detected by the sense amplifier. Thedecrease of remanent polarization depends upon the material properties of the ferroelectriccapacitor, pulse amplitude and width as well. Fatigue results are strongly affected by the degreeof switching caused by the fatigue excitation signal. For complete switching the fatigue behavior is independent of the fatigue frequency and only the number of cycles is decisive for the decreasein polarization.[2.8]

2.5.2 Retention Loss

Retention loss is the decrease of polarization after long term storage. As the ferroelectric cannotideally retain the remanent polarization because of depoling and backswitching similar to fatigue

the difference between switching and non-switching charge becomes smaller. The retention lossis determined by measuring the retained charge, e.g. by means of a negative read pulse, after acertain period of time when the state was stored by a positive wrote pulse.[2.10], [2.8]

Figure 2.7 Pulse scheme for measuring the retention loss in ferroelectric capacitors andhysteresis loop[2.10]

2.5.3 Imprint

Imprint is the tendency of one polarization state to become more stable than the other state, which

accompanies the loss of polarization. Imprint failure is the crucial degradation factor which iscaused by maintaining a remanent polarization for a long time which causes the shift of ferroelectric hysteresis loop with respect to the applied voltage. Due to imprint, the remanent

polarization decays as a function of storage time. Imprint affects the ferroelectric behavior in twoways [2.4].

A shift of ferroelectric hysteresis loop on the voltage axis is observed (memory device becomesnon-switchable) and on the other hand it leads to a loss of remanent polarization.(due to which



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the sense amplifier is unable to distinguish between two different pola r ization states) Interfacescreening mechanism proposed that imprint is due strong electric field at the interface betweenthe thin film and the electrode this field is due to the thin surface layer in which the spontaneous

polarization is reduced or even absent.

3.1 Phase Change Random Access Memory (PRAM)

PRAM or Phase Change Random Access Memory or also referred us as Phase change Memory(PCM) uses the behavior of the Chalcogenide Glass. Chalcogenide is a material used in re-writeable CD’s and DVD’s .The major property of this material due to which it is used inmemory devices is that this material change his phase reversibly and quickly between anAmorphous and poly crystalline state. The Crystalline and Amorphous states of Chalcogenidehave dramatically different Electrical Receptivity values, and this forms the basis by which data

is stored. The Amorphous, high resistance state is used to represent a binary 1, and theCrystalline, low resistance state represents a 0.

3.2 Effects and Materials: PRAM

PRAM is not a new idea actually this idea is dated back to 60’s when Stanford Ovshinsky firstexplored the properties of the Chalcogenide Glass. He studied the properties of the materialswhich change the phase when the temperature is changed

3.2.1 Potential Materials and Effects

Chalcogenide Glass has the ability to change its phase from Crystalline to Amorphous when thetemperature is changed. The basic material for the phase change memory is GeSbTe (GST) alloyand is the same family of material. Below some phase change alloys are mentioned.

Binary Ternary Quaternary

Ga SbIn SbIn SeSb2 Te3

Ge Te

Ge2Sb2Te5

In Sb TeGa Se TeSn Sb2 Te4

In Sb Ge

Ag In Sb Te(Ge Sn)Sb TeGe Sb (Se Te)Te81Ge15Sb2S2

Ovshinsky discovered the fact that the Chalcogenide materials have the reversible phase changewhere a material changes from amorphous phase to a Crystalline Phase, by the support of jouleheating. During phase change the material is heated above the melting point and this point theheating is stop and the material is quenched quickly so that the material is in amorphous state asthe material has reversible so if the phase change material has to be heated up above theCrystallization temperature but less than the melting temperature so the material can crystallize.This effect already used in CD’s and DVD’s where a laser heats up local spots. The different



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phases have different diffractive indexes; because of this reason different states written can berespectively read out. The 2 ovonic effects are observed in nickel oxide, transition metal oxide,amorphous silicon, metal semiconductor structures, chalcogenide and tantalum [3.1].All of theabove materials have phase change properties but in most cases Ge 2Sb2Te5 is used and normallyit refer us to GST and this material is more studied than any other of its family[3.1]

3.2.2 Crystal Structure of GST(Ge2Sb2Te5 )

GST has two type of crystal structure first is and FCC and the other is stable hexagonal butregarding PRAM only FCC crystal is observed and the reason is that it crystallizes faster asshown in figure 3.1, where all atoms have a six-fold coordination. It is observed that dependingon the stoichiometry Ge-Sb sub lattice has about 20% free vacancies. Compared to theAmorphous GST (figure 3.2) all atoms are bondedAccording to their natural valence, this means that germanium has four-fold coordination,antimony has threefold bonding and tellurium two-fold bonds

[3]

Figure 3.1 lattice of crystalline GST with 20%Vacancies

Figure 3.2 lattice of amorphous GST dashed lineshows possibility of Tellurim bond (2)

3.2.3 Why (Ge2Sb2Te5) ?

This material has been studied more then other material of the same family it is due to some properties which distinguish him from other phase change material. It is a ternary compound of Germanium, Antimony and Tellurium .its crystallization times is 20ns and allows writing bitrates

up to 35 M/bits and direct overwrites capability of upto 105 cycles. It is also used in rewritableDVDs. Other similar material is e.g. AgInSbTe It offers higher linear density, but has lower overwrite cycles by 1-2 orders of magnitude. It is used in groove-only recording formats [3.4]

3.3 Implementation of PRAM

3.3.1 Voltage, current waveforms and temperature for the set and reset

operation in GeSbTe:-



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The effect of Ovonic phase change can be used for the phase change memory .Because the idea behind such kind of memories is the difference in resistance. The low resistance is representingthe crystalline state while the high resistance is represented by amorphous [3.6]. We can see fromthe fig to write amorphous state the current has to reach 1.0mA to melt the material. After quicklycooling the material as a result the amorphous phase is programmed. To crystalline phase the

current has to be driven upto 0.6 mA. Read out occur by applying a voltage less than Vth, becausethe resistance difference between two phases is large enough to be detected

3.3.2

3.3.2 Current Status of PRAM

A lot of research is going on to develop commercially available PRAM and some ground breaking success we hear for last couple of years. In reality the big market leaders of

semiconductor industry are directly competing with each other for the early success. Last year information technology giant IBM announces their alliance with German company Siemens for the joint research on PRAM [3.15].

Last year A team from Philips Research (Eindhoven, The Netherlands) is due to publish detailsof a Chalcogenide non-volatile phase-change memory cell made from doped antimony-telluridewhich has a low threshold voltage and should therefore scale with future integrated circuitmanufacturing processes. For a 50-nanometer long strip of phase-change material the requiredvoltage is 0.7-V — a good match to the voltage that future silicon chips are expected to be able to



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provide, Philips said. In addition the phase changes occur extremely quickly, typically within 30nanoseconds in Philips.[3.7]

2:- Intel which is the market leader of such memory devices with its flash memory device alsotrying to develop phase change memory and for that Intel and STMicroelectronics investigate

phase change memory as successor to the flash memory and both leading semiconductor

companies sign an agreement dated (06/09/2006 10:40 AM EDT

3:- The first prototype of the phase change memory showed by the Samsung last month. The newmemory technology is currently under heavy research from Samsung, but the company has finallydemonstrated a working 512 megabit sample. According to Samsung, PRAM is slated to replacecurrent NOR flash memory technology within the next several years [3.8]

3.4 Major Challenges for PRAM

Even this technology assumed to be the future of the memory cells the major problem with this

technology is high reset current. We need this current because of low resistance of the crystalline phase. The programming current is typically larger then 1mÅ but there is need to cut down thecurrent for practical application. There are some ideas available to reduce the reset current likeedge current [3.12]. An other idea is to dope the chalcoganide with nitrogen [3.13] based onresearch work on rewritable optical memory disks. It is because if doped with nitrogen result as ahigher resistance material so it tends to reduced the current

Normally nitrogen can occupy the vacancies in tetrahedral intensities of the FCC structure .thesize of the nitrogen is smaller as compare to that of GST but this is an assumption and the other assumption is that doping of nitrogen ,which does not occupy vacancies but creates nitrides likeSb-N, Ge-N and Te-N and condense into grain and due to condensation of nitrides the grainunable to grow during crystallization so that smaller grain size correspond to higher resistance for electron .so automatically the reset current decreases [3.13]

.

4 CONCLUSIONS

There are nearly 30 options on which scientist are working to replace the traditional flashmemory. Only few of them can be compared because most of them are only proposals in theresearch papers or may be at very beginning stage . So there only few of them have competitionto each other and that are MRAM, FeRAM and PRAM. Major properties for all these RAMs has

been summarized in the following table [3.11]



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The flash memory has two big disadvantages of low endurance and high value of write time. If we discuss about FERAM it consists of ITIC which use SBT/PZT as a dielectric material. For thistechnology small embedded memories are available. They have very reduced cell density. But themain problem with FERAM is its complicated process and some of its unsolved problems likeimprint and retention loss. PRAM has a very serious problem of high programming currentswhich is very important issue with respect to handheld devices because they have very limited

power.MRAM has similar speeds to SRAM, similar density but much lower power consumption thanDRAM, and is much faster and suffers no degradation over time in comparison to Flash memory.It is this combination of features that some suggest make it the "universal memory", able toreplace SRAM, DRAM and Flash. This also explains the huge amount of research being carried

out into developing it. The only commercial product widely available at this point is FreescaleSemiconductor's 4 Mbit part, produced on a several-generations-old 180 nm process [1.14]

5 REFERENCES

[1.1] L.L. Hench, J. K. West, “Principles of Electronic Ceramics”, John Wiley & Sons

[1.2] http://physics.unl.edu/~tsymbal/tsymbal_files/page0002.html

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

[1.4] Rainer Waser, “ Nanoelectronics and Information Technology: Advanced ElectronicMaterials and Novel Devices”, Wiley VCH 2003



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[1.5] http://www.mram-info.com/

[1.6]http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MR2A16A&nodeId=015424

[1.7] W. J. Gallagher and S. S. P. Parkin, Development of the magnetic tunnel junction MRAM at

IBM: From first junctions to a 16-Mb MRAM demonstrator chip

[1.8] J. M. Daughton, “Magnetoresistive Memory Technology,” Thin Solid Films 216, No. 1,162–168 (1992).

[1.9] R. Meservey and P. M. Tedrow, “Spin-Polarized Electron Tunneling,” Phys. Rep. 238, No. 4, 173–243 (1994).

[1.10] J. Nowak and J. Rauluszhkiewicz, “Spin Dependent Electron Tunneling BetweenFerromagnetic Films,” J. Magn. Magn. Mater. 109, 79–90 (1992).

[1.11] T. Miyazaki and N. Tezuka, “Giant Magnetic Tunneling Effect in Fe/Al2O3/Fe Junction,” J. Magn. Magn. Mater. 139, No. 3, L231–L234 (1995).

[1.12] T. M. Maffitt, J. K. DeBrosse, J. A. Gabric, E. T. Gow, M. C. Lamorey, J. S. Parenteau,D. R. Willmott, M. A. Wood, and W. J. Gallagher, “Design Considerations for MRAM,” IBM J.

Res. & Dev. 50, No. 1, 25–39 (2006, this issue).

[1.13] S. A. Wolf, A. Y. Chtchelkanova, and D. M. Treger, Spintronics—A retrospective and perspective

[1.14] http://en.wikipedia.org/wiki/Mram

[2.1] Ali Sheikholeslami, MEMBER, IEEE, AND P. Glenn Gulak , SENIOR MEMBER, IEEE,”A Survey of

Circuit Innovations in Ferroelectric Random-Access Memories”

[2.2] D. N. Nguyen, Member, IEEE, and L. Z. Scheick, Member, IEEE, “TID Testing of Ferroelectric Nonvolatile RAM”

[2.3] http://en.wikipedia.org/wiki/Ferroelectric_RAM

[2.4] http://www.rci.rutgers.edu/~ecerg/projects/ferroelectric.html

[2.5] http://www.fujitsu.com/sg/

[2.6] http://www.sou.edu/physics/ferro/nsf_wht.htm

[2.7] L.L. Hench, J. K- West, “Principles of electronic ceramics”, John Wiley & Sons

[2.8] Keum Hwan NOH_, Beelyong YANG1, Seok Won LEE, Seaung-Suk LEE, Hee-Bok KANG and Young-Jin PARK, “Issues and Reliability of High-Density FeRAMs”

[2.9] http://www.msm.cam.ac.uk/doitpoms/tlplib/ferroelectrics/index.php



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[2.10] Rainer Waser, “ Nanoelectronics and Information Technology”, Wiley VCH

[3.1] Roberto Bez, Agostino Pirovano “Non-volatile memory technologies: emerging conceptsand new materials” Materials Science in Semiconductor Processing 7 (2004) 349–355

[3.2] Agostino Pirovano et al “Electronic Switching in Phase-Change Memories” IEEE

Transaction on Electron Devices Vol. 51, No. 3 pp. 452-459 March 2004

[3.3] André Kalio 12 th Juni 2005

[3.4] www.wikipedia.com

[3.5] Jin Ho Oh (Ph.D Candidate) Research Topic : Chalcogenide Thin Film Research for PRAMand ReRAM ApplicationSeung Wook Ryu (M.S. Candidate) Research Topic : Investigation on the phase change propertyof Ge-Sb-Te thin film

[3.6] :- Woo Yeong Cho et al “A 0,18µm 3V 64Mb Non-volatile Phase transition PRAM” IEEE

Journal of Solid-State Circuits, Vol. 40, No. 1, pp. 293-300, January 2005

[3.7] Philips research www.philips.com

[3.8] www.samsung.com press release

[3.9] S. D. Savransky, Engineering of Creativity: Introduction to TRIZ Methodology of InventiveProblem Solving, CRC Press, 2000, 408 pp

[3.10] Agostino Pirovano “Reliability Study of Phase-Change Non-Volatile Memories” IEEE Transactions on Device and Materials Reliability Vol. 4, No. 3, pp. 422-427 September2004

[3.11] Roberto Bez, Agostino Pirovano “Non-volatile memory technologies: emergingconceptsand new materials” Materials Science in Semiconductor Processing 7 (2004) 349–355

[3.12] Y.H. Ha, J.H. Yi, H. Horii, J.H. Park, S.H. Joo, S.O. Park,U-In Chung, J.T. Moon, “AnEdge Contact Type Cell forPhase Change RAM Featuring Very Low PowerConsumption”,Symposium on VLSI Tech Digest of TechPapers, 2003.

[3.13] H. Horii, J.H. Yi, J.H. Park, Y.H. Ha, I.G. Baek, S.O.Park, Y.N. Hwang, S.H. Lee, Y.T.Kim, K.H. Lee, U-InChung, J.T. Moon, “A Novel Cell Technology Using NdopedGeSbTe Filmsfor Phase Change RAM”, Symposiumon VLSI Tech Digest of Tech Papers, 2003.

[3.14] Y.N. Hwang, J.S. Hong, S.H. Lee, S.J. Ahn, G.T. Jeong,G.H. Koh, J.H. Oh, H.J. Kim,

W.C. Jeong, S.Y. Lee, J.H.Park, K.C. Ryoo, H. Horii, Y.H. Ha, J.H. Yi, W.Y. Cho, Y.T.Kim,K.H. Lee, S.H. Joo, S.O. Park, U.I. Chung, H.S. Jeong,Kinam Kim, “Full Integration andReliability Evaluation ofPhase-change RAM based on 0.24 µm-CMOSTechnologies”,Symposium on VLSI Tech Digest of TechPapers, 2003.

[3.15]http://www.ovonic.com/PDFs/annual_meeting_111505/lowrey_stockholder_meeting_111505.pdf (annual meeting 2005)

microprocessor chapter 6 ram

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