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JOURNAL OF APPLIED PHYSICS VOLUME 94, NUMBER 5 1 SEPTEMBER 2003~13 mA!, the slow set writing speed ~>50ns!, and the thermal fatigue of the phase change material.The operation current level should be reduced to a few hun-dreds of mA for low-power high-density memory chip pro-duction.

Among several cell structures that have been developedfor PCRAMs, the 1 transitor/1 resistor ~1T/1R! structure withcontact plug,4,6 shown in Fig. 1, has the strongest potentialfor mass production since it is relatively simple and pos-

as a resistive heater increasing the temperature above theglass transition temperature (Tt) or the melting temperature(Tm); also it provides a conducting path for the current re-quired for read and write operations. For this reason, the plugmaterial in contact with the Ge2Sb2Te5 layer is often called aresistive heater. As far as we are aware, no work has yet beenreported on the requirements for a resistive heater in thiscontext even though several candidates for resistive heatermaterials such as doped polysilicon,2 TiN,6,7 and TiAlN ~Ref.8! have recently appeared in literature.

It is important to calculate the temperature distribution ina memory cell for several plug materials having differentthermal and electrical properties, since such information ishelpful for choosing the right resistive heater. In this study, aone-dimensional heat conduction model is developed in or-der to simulate the writing operation in the 1T/1R PCRAMcell with plug contact. Based on this model, the reset currentvalues are calculated for various plug systems and comparedwith the experimental data of other researchers.4,68 It isshown that the insertion of an additional heating layer be-tween the TiN and Ge2Sb2Te5 layers makes a reset operationpossible at a current level of 12 mA, suitable for low-current operation. Finally, the requirements of such an addi-tional heating layer are discussed in terms of its electricala!Electronic mail: [email protected] heat conduction morandom access memory device with a

Dae-Hwan Kang, Dong-Ho Ahn, Ki-Bum Kim,a) JResearch Institute of Advanced Materials, Seoul National Uand Engineering, San 56-1, Shillim-dong, Kwanak-ku Seou

~Received 28 March 2003; accepted 11 June 2003!

A one-dimensional heat conduction model is developedevice with an 8F2 memory cell structure (F50.15operation, which corresponds to the phase switching fphase ~0 state! of Ge2Sb2Te5 , was investigatedprofiles for the memory cell structure. It is revealedcurrent level ~2 mA! reported for existing devices withis used as a resistive heater. However, it is possiblthickness is inserted between the TiN and Ge2Sb2Te5is higher than 105 mV cm, and the thermal conductivGe2Sb2Te5 . In addition, it is shown that a reset operealized in this memory cell when amorphous carboused as an additional heating layer. It is believed thaconduction model is a useful tool for analyzing the dememory devices and for selecting the proper conditiolow-current operation. 2003 American Institute o

I. INTRODUCTION

The phase change random access memory ~PCRAM! is apossible substitute for all kinds of current memory devicessuch as dynamic random access memory ~DRAM!, staticrandom access memory, flash memory, and others. ThePCRAM has a simple cell structure with high scalability; it isnonvolatile, has a relatively high read/write operation speed3530021-8979/2003/94(5)/3536/7/$20.00

Downloaded 23 Aug 2003 to 147.46.69.90. Redistribution subject tel for an electrical phase change8F2 memory cell F0.15 mm

F. Webb, and Kyung-Woo Yiiversity, School of Material Science

151-742, Korea

for a phase change random access memorym). The required current level for a resetm a crystalline ~1 state! to an amorphousy calculating one-dimensional temperatureat a reset operation is not achieved at thesubquarter micron plug size when only TiN

when an additional heating layer of 5 nmyers, for which the electrical resistivity releck and specific heat c are as low as those of

tion at a low current level of 1 mA can be(k50.2 W/m K and relec5106 mV cm) isthis relatively simple one-dimensional heatce operation of phase change random accessfor an additional heating layer allowing for

Physics. @DOI: 10.1063/1.1598272#

sesses a similar cell structure to that of a conventionalDRAM. In addition, it is possible to operate at a low switch-ing current of the order of milliamps since phase switchingonly occurs in a small region ~a programmable volume asshown in Fig. 1! above the plug material. In such a 1T/1Rcell with a plug, it is believed that the selection of the rightplug material is a key to low-current operation because it acts6 2003 American Institute of Physics

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resistivity, thermal conductivity, specific heat, and meltingtemperature values.

II. ONE-DIMENSIONAL HEAT CONDUCTION MODELDESCRIPTION

We consider a one-dimensional heat conduction modelin order to predict how much external current is necessaryfor phase switching of Ge2Sb2Te5 from a crystalline to anamorphous phase in the 1T/1R PCRAM cell structure. Twodifferent currents are necessary for PCRAM operation. Oneis a reset current for switching from a crystalline to an amor-phous phase and the other is a set current which switchesfrom an amorphous to a crystalline phase. The reported val-ues of set and reset currents for a 1T/1R cell structure, witha subquarter micron plug size, have a range of 0.51.5 mAand 1.52.5 mA, respectively.4,6,7 Therefore, the reduction ofthe reset current level is more important for low-current op-eration, and we will focus on this. It should also be notedthat it is sufficient to consider a one-dimensional model sinceboth current flow and heat dissipation mainly occur in the xdirection shown in Fig. 1, although a multidimensionalmodel is necessary for a more accurate simulation.

Figure 2~a! shows the schematic cross-sectional diagramof the memory cell for which the one-dimensional heatconduction model has been developed. The memory cellis assumed to have a Si-substrate/W-plug/resistiveheater/Ge2Sb2Te5/TiN/W layered structure, and the thicknessof each layer, not including the substrate, is 0.5, 0.05, 0.1,0.05, and 0.5 mm, respectively. Here, the TiN and W layersare taken to be a diffusion barrier and a metal plug material,respectively. In addition, we assume that the current state-of-the-art 0.15 mm DRAM technology with a conventional 8F2cell area is used to fabricate the PCRAM device; then thecross-sectional area of the plug contact (Aplug) is F2 ~0.0225mm2!, and the cross-sectional area of the Ge2Sb2Te5 layer

2 2

FIG. 1. Schematic diagram of a 1 transistor/1 resistor ~1T/1R! structure withcontact plug, showing a unit cell of a phase change random access memorydevice @from Refs. 4 and 6#.

J. Appl. Phys., Vol. 94, No. 5, 1 September 2003(AGST) is 3F ~0.0675 mm ! where F is a feature size of 0.15mm, as shown in Fig. 2~b!.

Downloaded 23 Aug 2003 to 147.46.69.90. Redistribution subject tThe phase switching of Ge2Sb2Te5 is triggered by Jouleresistive heating (qJoule), given by

qJoule5IF2 Rt , ~1!

where, IF is the external forcing current, R is the electricalresistance of the Joule heating material, and t is the heatingtime. Also, R is given by R5relec,/A , where relec is elec-trical resistivity, , is the length, and A is the cross-sectional

FIG. 2. ~a! The schematic cross section of the PCRAM memory cell forwhich a one-dimensional heat conduction model is developed and ~b! thetop view of a conventional 8F2 memory cell.

3537Kang et al.area of the Joule heating material, respectively. The one-dimensional temperature profile versus the distance x from


the active region of the Si substrate can be obtained by solv-ing the one-dimensional heat equation with a source term ofthe form qJoule . For the ith layer, the equation is

r i]Ti]t

5k ic i

]2Ti]x2

1qJoule,i

c i, xi2199~ARc1rc, !

ra2rc5xcrit , ~4!

where x is the programmable distance, , is the total length,and A is the cross-sectional area of the Ge2Sb2Te5 layer; Rcis the total circuit resistance, excluding that of the Ge2Sb2Te5layer, and is the sum of the channel resistance of the celltransistor RTr , source/drain resistance RSD , plug resistanceRplug , metallization resistance Rmetal , and a series of contactresistances Rcontact . It has been found that Rc has a range of10001500 V under typical current ~0.30.5 mA! and volt-age ~0.5 V! conditions for a read operation.4,7,8 In addition,

3538 J. Appl. Phys., Vol. 94, No. 5, 1 September 2003rc and ra are the electrical resistivities of the crystalline ~416mV cm!11 and amorphous ~>100 V cm!12 phases of

Downloaded 23 Aug 2003 to 147.46.69.90. Redistribution subject tGe2Sb2Te5 , respectively. Taking Rc to be 1500 V, xcrit is 93. Therefore, the programmable distance for a read operationshould be greater than 1/10 of the total thickness ~1000 ! ofthe Ge2Sb2Te5 layer in the structure depicted in Fig. 2~a!. Asa result, the critical point (xcrit ,Tm) for a reset operation,calculated from the heat conduction model, is 0.56 mm,632 C. It is concluded that a write operation is only possiblewhen the temperature profile at x50.56 mm generated by areset current is greater than or equal to 632 C. Based on thisone-dimensional heat conduction model, it is possible to es-timate the reset current value for various plug systems andcompare it with experimental data of other researchers;4,68this will be described in Sec. III.

III. RESET CURRENT CALCULATIONThe one-dimensional heat equation, Eq. ~2!, is solved

using numerical techniques employing an implicit time stepand a second-order finite central difference scheme in space.It is noted that the temperature dependencies of physicalproperties such as thermal conductivity, specific heat, andelectrical resistivity are neglected and we use values at 25 Cin the temperature profile calculation. However, when solv-ing the equation, we take into account the heat loss duringthe phase transformation of Ge2Sb2Te5 from a crystalline toa liquid phase at 632 C, which is an endothermic process sothat some of the Joule heat is consumed by the heat of fusion~622 J/cm3 for Ge2Sb2Te5).

First of all, consider the memory cell of Fig. 2~a! inwhich a TiN layer only is used as a resistive heater, such acell is currently being evaluated for its suitability as aPCRAM memory cell.6,7 The required thermal and electricaldata for W, Ge2Sb2Te5 , and TiN at 25 C are given in Table

FIG. 3. ~a! The resistance of a set state RSET state and ~b! reset state RRESETfor the read operation of the PCRAM. Here, Rc is the circuit resistance, rcand ra are the electrical resistivities of the crystalline and amorphous phasesof Ge2Sb2Te5 , respectively, x is the programmable thickness, , is the totallength, and A is the cross-sectional area of the Ge2Sb2Te5 layer.

Kang et al.I. Note that TiN has a wide range of electrical resistivity andthermal conductivity values. It is well known that the resis-


FIG. 4. Temperaturesubstrate for differentthe layered structure:

Te

ificJ/g

.13

.20

.78

v

Downloaded 23 profiles vs distance x from the active region of the Sitivity of a TiN film ranges from several tens of mV cm ~forhigh-quality sputtered TiN films!13 to thousands of mV cm~for low-quality chemical vapor deposited TiN films!.14,15 Sothe value depends on the film composition, impurity level,and deposition technique. However, since the TiN layer isregarded as a metallic conductor, the thermal conductivitiesof TiN in Table I are estimated using a WiedemannFranzlaw which states that the electrical and thermal conductivi-ties of metals are proportional at a given temperature.

Figure 4 shows the calculated temperature profiles for aSi-substrate/W-plug/TiN-heater/Ge2Sb2Te5 /TiN/W structureat different resistivities and thermal conductivities of TiN.The current is fixed at 2 mA, a typical value for a resetcurrent. It is noted that the temperature in the Ge2Sb2Te5layer slightly increases as the resistivity of TiN is increasedbut it is as low as 180 C even with a resistivity of 1000mV cm and thermal conductivity of 0.44 W/m K. These re-sults indicate that for a memory cell with only a TiN heater,a reset operation is impossible because its temperature pro-file does not encompass the critical point ~0.56 mm and632 C!. It is believed that such a low temperature is due tothe relatively low electrical resistivity and high thermal con-ductivity of TiN. We can conclude that a good electrical andthermal conductor, like TiN, is not able to generate enoughJoule heat while at the same time rapidly dissipating the heatin the Ge2Sb2Te5 layer to the surroundings.

TABLE I. Reported physical properties of W, Ge2Sb2

Meltingtemperature

Tm ~C!

Mass densityr (3106 g/m3)

Specc ~

W 3407 19.3 0Ge2Sb2Te5a 632 6.2 0

TiN 2950 5.24 0

aSee Ref. 8.bSee Ref. 11.cAssumed values from the resistivity ~or conductivity!

J. Appl. Phys., Vol. 94, No. 5, 1 September 2003values of TiN resistivity and thermal conductivity inSi substrate/W plug/TiN heater/Ge2Sb2Te5/TiN/W.

Aug 2003 to 147.46.69.90. Redistribution subject tFor effective Joule heating and heat isolation of theGe2Sb2Te5 layer, let us introduce an additional heating layerbetween the TiN heater and Ge2Sb2Te5 , so that the structurebecomes a Si-substrate/W-plug/TiN-heater/additional heatinglayer/Ge2Sb2Te5 /TiN/W, as shown in Fig. 5. The thicknessof the additional layer is assumed to be 5 nm. Here, a pre-requisite is to determine what range of electrical and thermalproperties an additional heating layer should have, especiallycompared with those of Ge2Sb2Te5 . They are electrical re-sistivity (relec), thermal conductivity ~k!, specific heat ~c!,and mass density ~r!, as shown in Eqs. ~1! and ~2!.

Figure 6~a! shows the calculated temperature profiles inthe memory cell in Fig. 5 with different resistivities for theadditional heating layer assuming other physical properties,such as thermal conductivity, mass density, and specific heatto be the same as those of Ge2Sb2Te5 . When the resistivityof an additional heating layer is below 105 mV cm, the tem-perature profiles at x5xcrit are much lower than the value Tmrequired for a reset operation. This indicates that the Jouleheating is still insufficient to increase the temperature aboveTm in the Ge2Sb2Te5 layer. However, with a heater-layerresistivity of 105 mV cm, the temperature at the interfacebetween the heater and Ge2Sb2Te5 layer reaches Tm and itrapidly increases as the resistivity is increased further.

5 , and TiN at 25 C.

heatK!

Thermal conductivityk ~W/m K!

Electrical resistivityrelec ~mV cm!

2 174 5.392 0.46 416b4 220.44c 201000

alues of TiN film by using a WiedemannFranz law.

3539Kang et al.FIG. 5. Schematic cross section of the memory cell when an additionalheating layer is inserted between the TiN heater and Ge2Sb2Te5 layers.


FIG. 6. Temperature profiles vs distance x from the active region of the Sisubstrate in a memory cell with a TiN-heater/additional heating layer(5 nm)/Ge2Sb2Te5 structure for heating layers with different values of ~a!

3540 J. Appl. Phys., Vol. 94, No. 5, 1 September 2003electrical resistivity, ~b! thermal conductivity, and ~c! specific heat, wherecurrent IF is fixed at 2 mA.

Downloaded 23 Aug 2003 to 147.46.69.90. Redistribution subject tNext, by fixing the electrical resistivity at 105 mV cm,the temperature profiles were calculated for additional heat-ing layers with various thermal conductivities in order to findthe optimum range; these are plotted in Fig. 6~b!. In thiscase, the mass density and specific heat are assumed to be thesame as for Ge2Sb2Te5 . The temperature at x5xcrit fails toexceed Tm when the thermal conductivity of an additionalheating layer is higher than that of Ge2Sb2Te5 ~0.46 W/m K!.However, the temperature at the top (x50.555 mm) of theadditional heating layer starts to rise above Tm when thethermal conductivity is equal to or lower than that ofGe2Sb2Te5 . As is seen in Fig. 6~c!, the effect of the specificheat on the temperature profiles is the same as that of thermalconductivity. That is, the temperature at x5xcrit is higherthan Tm when the specific heat is equal to or lower than thatof Ge2Sb2Te5 ~0.202 J/g K!. In contrast, the temperature pro-file was not affected by the change of density.

From these results, it is concluded that the electrical re-sistivity of an additional heating layer should be higher than105 mV cm and its thermal conductivity and specific heatshould be as low as that of Ge2Sb2Te5 for a 2 mA resetoperation for the memory cell structure of Fig. 5. In practice,it is difficult to find a material satisfying these requirementssimultaneously because electrical conductors generally havea high thermal conductivity. Nonetheless, amorphous carbon(a-C) can be considered as an additional heater layer since itis reported to have a low thermal conductivity range ~0.22.2W/m K!16 and an intermediate electrical resistivity range(104 106 mV cm),17,18 depending on its composition, bond-ing structure, or on the deposition techniques. The reportedphysical properties of a-C are summarized in Table II. Itsspecific heat is assumed to be the average value of the twocrystalline allotropes ~diamond and graphite!.

Figure 7~a! shows temperature profiles at 2 mA whena-C with different thermal conductivities16 is used as an ad-ditional heating layer. For convenience, its electrical resistiv-ity is fixed at 106 mV cm. It is clear that the temperature atthe interface between the a-C and Ge2Sb2Te5 layers ishigher than Tm . Furthermore, it is shown in Fig. 7~b! that areset operation at a low current of 1 mA can be realized in a0.15 mm 8F2 memory cell when a-C (k50.2 W/m K andrelec5106 mV cm) is used. Therefore, it is believed that a-Cis a good candidate for a resistive heater material for a lowoperating-current PCRAM device.

IV. DISCUSSION

Contemplating the results in Sec. III, it is believed thatthree factors should be taken into account when selecting aresistive heating layer for low-current PCRAM device opera-tion. The first one is that the Joule resistive heat should beefficiently generated by a low current. For this, a materialwith high electrical resistivity and low specific heat is pref-erable, as shown in Figs. 6~a! and 6~c!, respectively. Thehigher the electrical resistivity, the larger the amount of Jouleheat generated by the term in Eq. ~1!. In addition, the lowerthe specific heat, the higher the temperature for the same

Kang et al.Joule heating rate because less heat is required to increasethe temperature. However, there is an upper limit for the


9 10 eset state duringcrystalline phasesolid amorphousuction modelinge temperature oftuates somewhat. 8~b!.perature profilesfrom the one-FIG. 7. Temperatureture with various the

52 mA and ~b! IF51is fixed at 106 mV cm

s c

heK

a

/g

Downloaded 23 10 10 C/s is the key to achieving a rPCRAM device operation by melting theand quenching the liquid phase to form aGe2Sb2Te5 phase; this is why the heat condis important. However, it is not clear why ththe Ge2Sb2Te5 layer is not constant but flucduring the current-on period, as seen in Fig

Finally, it should be noted that the temin Figs. 4, 6, 7, and 8 were calculatedprofiles of TiN-heater/a-C (5 nm)/Ge2Sb2Te5 struc-rmal conductivities @from Ref. 16# for a-C at ~a! IFTABLE II. Reported physical properties of amorphou

Meltingtemperature

Tm ~C!

Mass densityr (3106 g/m3)

SpecificC ~J/g

a-C 36523697 1.82.1 0.617

aAssumed to be the average value of graphite ~0.714 JbSee Ref. 16.cSee Refs. 17 and 18.

J. Appl. Phys., Vol. 94, No. 5, 1 September 2003mA. For convenience, the electrical resistivity of a-C.

Aug 2003 to 147.46.69.90. Redistribution subject tresistivity, since if it is too high, the read operation on thememory cell fails due to its high resistance. Simple calcula-tions for the memory cell in Fig. 5 show that the electricalresistivity of an additional heating layer should not be greaterthan 106 mV cm so that its resistance does not exceed thetotal circuit resistance (Rc). The second factor is that it isimportant to prevent the generated Joule heat from beingdissipated through the contact plug region by making a re-sistive heating layer with a thermal conductivity as low asGe2Sb2Te5 , as shown in Fig. 6~b!. Third, the resistive heatershould have a higher melting temperature than that ofGe2Sb2Te5 ~632 C!, since it should not melt during deviceoperation.

a-C is a good candidate for a resistive heater satisfyingthe aforementioned requirements, as shown in Fig. 7. Someoxynitrides such as TiOxNy ~Ref. 19! and thin films of oxidessuch as Al2O3 ,20 in which current flows due to quantum-mechanical tunneling, may also be considered as materialssatisfying these requirements, since they are likely to havethermal conductivities in a range as low as 12 W/m K andan electrical resistivity range of 104 106 mV cm.

It is also important to investigate the temperature profileunder actual device operation conditions. In an actual resetoperation, a current pulse is applied to the memory cell in-stead of the constant current used in Figs. 4, 6, and 7. Forexample, consider a current pulse, shown in Fig. 8~a!, forwhich the on- and off-current periods are 50 ns. Figure 8~b!shows the resulting temperature profile versus distance x andtime t at 1 mA from our heat conduction model when such acurrent pulse is applied to the memory cell in Fig. 5 with aTiN-heater/a-C (k50.2 W/m K and relec5106 mV cm)/Ge2Sb2Te5 structure. It is notable that the temperature in theGe2Sb2Te5 layer rises above 1000 C after a short time ~20ns!. In addition, the temperature drops abruptly from near1000 C to below 80 C within 20 ns as soon as the currentgoes to zero; this is due to the rapid heat dissipation throughthe TiN and W layers which have high thermal conductivi-ties. Such fast heating and cooling rates in the range

arbon at 25 C.

at!

Thermal conductivityk ~W/m K!

Electrical resistivityrelec ~mV cm!

0.22.2b 104 106c

K! and diamond ~0.521 J/g K!.

3541Kang et al.dimensional heat conduction equation so that they are some-what different from the actual ones. A multidimensional heat


By introducing an additional heater layer of 5 nm thick-ness between the TiN heater and the Ge2Sb2Te5 layers, it hasbeen shown that a reset operation is possible at the required

3542 J. Appl. Phys., Vol. 94, No. 5, 1 September 2003 Kang et al.conduction model is being developed to obtain more realistictemperature profiles. Furthermore, the reset currents for vari-ous resistive heating materials will be experimentally mea-sured and compared with the model.

V. CONCLUSIONA one-dimensional heat conduction model has been de-

veloped for a PCRAM device with a 0.15 mm 8F2 memorycell structure in order to evaluate the required current levelfor a reset operation. When a TiN heater only is consideredwith a 0.15 mm contact hole, the temperature at the interfacebetween the TiN and Ge2Sb2Te5 layers is far lower than themelting temperature of Ge2Sb2Te5 ~632 C! even with a re-sistivity of 1000 mV cm and thermal conductivity of 0.44W/m K, making the write operation impossible. This is sobecause the TiN is a good electrical and thermal conductor.

FIG. 8. ~a! A current pulse of 50 ns width and ~b! the resulting temperatureprofile vs current-flow time t, and distance x for the memory cell of Fig. 5with a TiN-heater/a-C (k50.2 W/m K and relec5106 mV cm)/Ge2Sb2Te5structure with a 1 mA current.Downloaded 23 Aug 2003 to 147.46.69.90. Redistribution subject tcurrent level of 2 mA. Moreover, the model predicts that alow-current operation of 1 mA can be realized in a 0.15 mm8F2 memory cell when the thermal conductivity of an addi-tional heating layer is lower than that of Ge2Sb2Te5 and itselectrical resistivity is as high as 106 mV cm. It is thoughtthat such a memory cell with a heater layer with low-k andmedium-relec values is a good candidate for a low-currenthigh-density PCRAM device. Finally, it is believed that themodel is a useful tool for estimating the operation character-istics of PCRAM devices with various plug structures.

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~1998!.o AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp

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2003_강대환(JAP)