influence of the surface roughness of the bottom electrode on the resistive-switching...

Microelectronics Reliability xxx (2014) xxx–xxx

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Microelectronics Reliability

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Influence of the surface roughness of the bottom electrodeon the resistive-switching characteristics of Al/Al2O3/Aland Al/Al2O3/W structures fabricated on glass at 300 �C

http://dx.doi.org/10.1016/j.microrel.2014.07.0060026-2714/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +52 (222) 22663100.E-mail address: [email protected] (J. Molina).

Please cite this article in press as: Molina J et al. Influence of the surface roughness of the bottom electrode on the resistive-switching characteristicAl2O3/Al and Al/Al2O3/W structures fabricated on glass at 300 �C. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.07.006

Joel Molina ⇑, Rene Valderrama, Carlos Zuniga, Pedro Rosales, Wilfrido Calleja, Alfonso Torres,Javier DeLa Hidalga, Edmundo GutierrezNational Institute of Astrophysics, Optics and Electronics (INAOE), Electronics Department, Luis Enrique Erro #1, Santa Maria Tonantzintla, Puebla 72000, Mexico

a r t i c l e i n f o

Article history:Received 26 February 2014Received in revised form 3 July 2014Accepted 3 July 2014Available online xxxx

Keywords:ALDAl2O3

MIM capacitorMemristorResistive switchingBEOL

a b s t r a c t

Resistive-switching devices based on Metal–Insulator–Metal (MIM) structures have shown promisingmemory performance characteristics while enabling higher density of integration. Usually, these MIMdevices are fabricated using different processing conditions including high temperature thermal treat-ments that could lead to undesirable chemical reactions in the insulator material and at its interface withthe metals involved. In this work, we compare the electrical characteristics of MIM devices (fabricated onglass at 300 �C) that use aluminum or tungsten as bottom electrode (BE) in order to study the influence ofa highly reactive (aluminum) or inert (tungsten) metal electrode on the memory characteristics. Wefound that the switching characteristics of Al2O3 (from a high-resistance state HRS to a low-resistancestate LRS and vice versa), are highly dependent on the surface roughness of the BE, the thickness ofAl2O3 and the current compliance (CC) which limits the electron density flowing through both top/bot-tom electrodes.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, highly integrated MIM structures have shown theability to switch between two different conduction modes in theirinsulators (whether metal oxides or solid electrolytes) by promot-ing formation/rupture of conductive filaments and/or ion migra-tion between both electrodes after applying a potential differencein them [1–4]. This phenomenon of resistive switching is knownas the memristance (memory resistance) effect [5,6] and thestructures able to replicate it are known as Resistive Random-Access-Memory (ReRAM) devices. The memristance effect isdefined as the ability to change the resistance state of a thin dielec-tric or solid electrolyte material from a usually high resistancestate (HRS/OFF), to a low resistance state (LRS/ON) and vice versa;thus developing characteristic gate current–gate voltage (Ig–Vg)hysteresis loops. By properly controlling the resistive switchingcharacteristics of many MIM structures, they have the potentialto replace standard non-volatile memory technologies with higherperformance characteristics (faster writing/erasing speeds, longdata retention times, better endurance and ultra-low power con-

sumption) along with a simple MIM architecture able to producehighly dense memory arrays.

For ReRAM operation, there are two characteristic switchingmodes: unipolar switching, which means the resistive switchingdepends on the amplitude of the applied voltage but not on thepolarity; and bipolar switching, which means the resistive switch-ing relies on the polarity of the applied voltage [7,8]. Duringswitching, the HRS and LRS can be obtained by applying VRESET

and VSET voltage pulses respectively. Importantly, an initial VFORM

pulse is required to initiate the memory operation (forming the ini-tial LRS) and usually, VFORM > VSET > VRESET. Once these MIM devicesreach the LRS, it is important to set up a limit on the current flow-ing through these devices since a large current density could dam-age the device permanently (the memristance effect will be lost).Usually, two different current limits are set up for the unipolarswitching mode in which the formation of conductive filamentsin the oxide (LRS, promoted by VSET) is limited by a low currentcompliance in the measurement system, CCSET. Dissolution of theseconductive filaments in the oxide (HRS, promoted by VRESET) isenhanced by setting a high current compliance, CCRESET. Ingeneral, the correct setting of these electrical parameters(VFORM > VSET > VRESET and CCRESET > CCSET) enables a successful andreproducible operation of ReRAM devices based on MIM structures.

s of Al/

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2 J. Molina et al. / Microelectronics Reliability xxx (2014) xxx–xxx

On the other hand, even though the precise physics behind theReRAM operation mechanism is still not fully understood, someobservations [9–10] suggest that specific physical/chemical condi-tions of the electrodes/oxides play important roles for predictableresistive switching. This is important since we could use highlyreactive or highly inert metal electrodes (having variable metalwork functions) along with oxides having specific thermodynamicstabilities, band gap energies and therefore, different band-offsetsto the metal electrodes (combined all-together, they would pro-duce different conduction mechanisms prior to switching to a LRS).

In an effort to better understand the switching phenomena ofReRAM devices based on Al2O3, we present and compare the elec-trical, electronic and morphologic characteristics of MIM devicesbased on Al/Al2O3/Al and Al/Al2O3/W stacked structures depositedon glass and processed at 300 �C in order to assess their introduc-tion into Back-End-Of-Line (BEOL) processing. By varying someparameters during the MIM fabrication process (different evapora-tion rates for the metallic BE and variable Al2O3 thickness), and byusing different current compliances (CC) during memory operation,distinctive cyclic I–V characteristics can be correlated to those vari-ations in the fabrication process which in turn, produce differentcarrier conduction mechanisms just before the initial switchingto the LRS occurs.

2. Experimental

The sequential deposition of Al/Al2O3/Al and Al/Al2O3/Wstacked structures were done on cleaned glass slides (Corning,2947). These glass substrates have in average, a surface roughnessof 1.26 nm (after AFM measurements in areas of 10 lm � 10 lm),ideal for sequential deposition of stacked materials. Since the mostimportant regions of these MIM devices are the interfaces of itsgate oxide with the metal electrodes (BE and TE), the good surfacequality of a silicon substrate is not used at all. The aluminum ortungsten layers (used as BE) are 500 and 400 nm in thicknessrespectively, and they are used in two different MIM structures.The Al2O3 is 10 nm in thickness and is deposited by atomic-layerdeposition (ALD) at 250 �C. After lithography (for gate patterndefinition), the whole stack was annealed in N2 at 300 �C.

For the initial cleaning procedure, the glass slides weredegreased by sequential immersion in trichloroethylene (TCE)and acetone by 10/10 min, respectively, within an ultrasonic vibra-tor. Then, the slides were rinsed in deionized water (DI) by 10 min,and gently dried using an ultra-high purity N2 pistol blow. Alumi-num and tungsten were the first metal layers being deposited onalready cleaned glass slides and they were used as bottom elec-trodes in two different MIM devices. These aluminum and tungstenlayers were 500 and 400 nm in thickness and they were depositedby E-beam evaporation (Temescal BJD-1800 from Edwards) underultra-high vacuum conditions using a deposition rate of 1 Å/s forboth electrodes. After BE deposition, 10 nm of Al2O3 was depositeddirectly on these metals by ALD (Savannah-S100, from CambridgeNanotech) at 250 �C using H2O and Trimethyl-Aluminum (TMA) aschemical precursors. During Al2O3 deposition, the ALD chamberwas kept at 250 �C/200 m Torr of temperature/pressure for all100 deposition cycles. Right after ALD of Al2O3, all samples wereimmediately moved back to the e-beam evaporator and the evap-oration chamber was vacuumed down to 4 � 10�7 Torr in order tominimize the exposure time of the Al2O3 surfaces to oxygen pres-ent in the atmosphere of the clean room. After reaching proper vac-uum conditions, a relatively thick aluminum film �500 nm wasthen evaporated on top of Al2O3 with an evaporation rate of 1–2 Å/s. Aluminum was the last metal layer being deposited onAl2O3 and it was used as top electrode (TE) for both MIM devices.Once fully metalized, all samples were covered with positive pho-

Please cite this article in press as: Molina J et al. Influence of the surface roughnAl2O3/Al and Al/Al2O3/W structures fabricated on glass at 300 �C. Microelectro

toresist using standard spinning/baking conditions and exposed toan UV system (Karl Suss MA6) to define the gate patterns of theMIM structures. A gate capacitor area of 64 � 10�6 cm2 was usedfor all MIM devices under test. After photolithography process,the final Al/Al2O3/Al/Glass and Al/Al2O3/W/Glass stacks wereannealed in pure N2 (99.999% purity) at 300 �C in order to promotedensification of the gate oxide and its interfaces with both metalelectrodes. As noticed, the fabrication procedure is quite simplewhile the maximum processing temperature is 300 �C, ideal forintegration into BEOL processing. Fig. 1 shows the complete fabri-cation processing flow for the MIM structures along with a simpli-fied MIM’s schematic and memory array.

On the other hand, the surface roughness for the first metallayer (aluminum or tungsten as BE) was measured by atomic-forcemicroscopy (AFM by NanoSurf EasyScan-2) and by taking intoaccount 10 different spots for three samples having similar deposi-tion conditions. Finally, capacitance–voltage (C–V, 1 kHz – 3 MHz)and current–voltage (I–V) measurements were both obtained usinga Semiconductor Device Analyzer (SDA, B1500A from Agilent), andby taking into account 10 different samples for each measurementcondition. All electrical measurements were obtained at roomtemperature.

3. Results and discussion

3.1. Electrical characteristics of MISCAP using Al2O3 as gate oxide

First of all, and in order to obtain the electrical quality of theproposed insulator, it is important to test the electrical character-istics of only the Al2O3 used in both MIM devices. For this, wedeposited a thinner Al2O3 layer (6 nm in thickness, using the sameALD and annealing conditions) directly on hydrogen-terminatedn-type silicon and measured several Metal–Insulator–Semiconduc-tor capacitors (MISCAP). Fig. 2 shows the gate current vs. gate volt-age characteristics (Ig–Vg under accumulation, before and afterhard-breakdown), of 10 different MISCAP (Al/Al2O3/n-Si) until theultra-thin gate oxides reach breakdown. We notice a high unifor-mity in the I–V characteristics (including almost the same break-down voltage) because of the high quality of the ALD techniquethat produces excellent uniformity in the oxide thickness even atthe atomic level [11,12]. For MIM structures, any deviation fromthese ideal I–V characteristics could then be related to the metalsused as BE.

3.2. Bipolar resistive switching in Al/Al2O3/Al and Al/Al2O3/Wstructures

Fig. 3(a) shows the resistive switching characteristics of the Al/Al2O3/Al-MIM structure in semilog format. Each electrical test forthe same MIM device consists of applying a double sweep of volt-age. These initial I–V characteristics are similar to that of bipolarswitching. The first sweep (negative polarity, from �5 V down to0 V) shows that the memory device is initially in the LRS (ON,reaching the CC) and when the voltage is reduced in magnitude,the device then switches to the HRS. For the positive polarity, thedevice shows an opposite conduction behavior. Now, when thevoltage sweeps back to �5 V (to complete one full sweep cycle),the device is kept in the LRS (OFF). For a new full sweep cycle,the whole behavior is also observed although the VSET is nowreduced. Here, VFORM > VSET as expected, while the IOFF/ION ratio isaround 6 orders of magnitude, which is a large resistivity windowand quite useful in order to obtain long lasting and large endurancecycles during memory performance. In this case, the Nth cyclerefers to the 8th I–V sweep and we notice that the gate currentbefore VSET is kept lower compared to its original level (positive

ess of the bottom electrode on the resistive-switching characteristics of Al/n Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.07.006


Fig. 1. Process flow for fabrication of Al/Al2O3/Metal devices as well as schematics of final device and possible integrated arrays. The maximum processing temperature usedwas limited to 300 �C.

Fig. 2. Gate current vs. gate voltage (Ig–Vg) characteristics of a MIS system based onthinner Al2O3. Excellent uniformity in the electrical characteristics is observed forall the measured samples.

J. Molina et al. / Microelectronics Reliability xxx (2014) xxx–xxx 3

polarity) which is an indication of electron trapping. A linear fit ofthis I–V curve (before VSET is reached) to well known conductionmodels suggest that the main conduction mechanism is Poole–Frenkel. This implies that electrons are trapped at some localizedstates in Al2O3 and complete recovery of the oxide layer after the

Fig. 3. (a) Bipolar resistive switching of Al/Al2O3/Al structures. The IOFF/ION ratio for this sdistinctive transitions from the HRS to LRS during VFORM and VSET. CC = 100 mA.


forming condition is not possible. Fig. 3(b) shows the same datain normal scale so as to clearly show the distinctive transitionsfrom the HRS to LRS during VFORM and VSET.

Fig. 4(a) shows the resistive switching characteristics of the Al/Al2O3/W-MIM structure in semilog format. It is clear that, com-pared to the former MIM structure using aluminum as BE, tungstenproduce noisy I–V characteristics which could suggest incompletenucleation of Al2O3 on this metal. Here, incomplete nucleation ofAl2O3 means that the oxide has not been properly deposited byALD on the surface of the substrate (tungsten surface in this case)so that obtaining pinhole-free, uniform and continuous films is notpossible [11]. This effect usually happens when the precursorsused during ALD do not react properly with the initial substrateso that the oxide film may not nucleate at all or may nucleate onlyat particular defect sites on the initial substrate (this is especiallythe case when depositing ultra-thin films during ALD). In our case,proper nucleation of Al2O3 was obtained since we were able todetect both the LRS and most importantly, the HRS in our MIMstructures. Otherwise, incomplete nucleation of Al2O3 would pro-duce only the LRS in the I–V data since both electrodes (TE andBE) would be in direct contact. Additionally, even though tungstenis a highly inert metal as compared to aluminum, it has beenreported that Al2O3 does not have severe nucleation problemswhen deposited on tungsten since complete nucleation requires

tructure is slightly higher than 106. (b) Same data in normal scale so as to show the



Fig. 4. (a) Bipolar resistive switching of Al/Al2O3/W structures. The IOFF/ION ratio for this structure is slightly higher than 104. (b) Same data in normal scale so as to show thedistinctive transitions from the HRS to LRS during VFORM and VSET. CC = 10 mA.


only one full ALD cycle [13,14]. Therefore, the physical and/orchemical origin of these instabilities is important in order to engi-neer processing solutions for this problem and thus, obtain repro-ducible I–V cycles during ReRAM operation. For the Nth (3rd) cycle,and around +2 V, we notice a sudden increase in Ig produced by asoft-breakdown event (promoting a localized breakdown of thegate oxide) that in this case, does not reach the maximum gate cur-rent limited by CC. The evidence for localized breakdown eventsare better visualized during the post-breakdown characteristicsof MIS devices, where the gate current levels (after breakdown)are directly related to localized leakage spots and which do nothave a direct correlation to the gate area of the devices [15,16].On the other hand, the window for resistance switching (IOFF/ION

ratio) is still large and about 4 orders of magnitude after the firstI–V cycle. Fig. 4(b) shows the same data in normal scale so as toclearly show the distinctive transitions from the HRS to LRS duringVFORM and VSET. There, we notice a larger I–V slope in the third cyclewhich suggests an increment in the total conductivity of this sys-tem when LRS is reached.

As noticed, Figs. 3 and 4 shows the I–V characteristics for MIMstructures using Al and W as bottom electrodes and the same phys-ical thickness for Al2O3. Even though the current compliance usedin those devices was slightly different (100 mA for aluminum and10 mA for tungsten), using the same CC produced similar I–V dataalthough with noisier characteristics and with less I–V cycles.Therefore, those levels for CC were chosen in order to present themore typical and reproducible I–V results from our samples. Whatis important here, is the broad change in the resistivity of Al2O3

(around 4-6 orders of magnitude) during bipolar operation of bothdevices. Also, given that noisier I–V characteristics are obtained forthe MIM devices using tungsten as BE, Fourier-transformed infra-red (FTIR) or X-ray photoelectron spectroscopy (XPS) analysis toAl2O3 and its interface with this electrode are needed in order tocorrelate this I–V behavior to its physical/chemical origin. This isimportant since by observing Figs. 3 and 4, we notice that the elec-trical behavior of the I–V characteristics is quite different eventhough the surface roughness of their bottom electrodes is similar(7.5 nm for aluminum and 5.8 nm for tungsten). Therefore, notonly the surface roughness is the single parameter influencing I–V behavior, but also, the chemical reactions occurring in theAl2O3 film when deposited on highly reactive or inert metallic sur-faces. As an additional note, the surface roughness of Al2O3 whendeposited on aluminum and tungsten was in average, 2.2 and1.4 nm respectively (after AFM measurements).

On the other hand, even though full reliability characteristics ofboth MIM structures are not presented because of the very limitedresistive switching cycles in our samples, we notice some impor-tant differences in the performance of these structures. As stated


earlier, the Al/Al2O3/W structure presents the noisier I–V charac-teristics which will compromise its performance and reliability.Now, by considering the inert quality of the tungsten electrodeand its work function (see its energy band diagram in the insetof Fig. 5b), we think that this last stacked structure is better suitedfor performance under lower voltage conditions. In the end, a reli-able switching operation is desired and since by using tungsten asBE results in soft-breakdown events, an Al/Al2O3/Al stack is desiredat this stage because earlier breakdown can be avoided duringoperation of this device.

3.3. Unipolar resistive switching in Al/Al2O3/Al structures and C–Vcharacteristics of Al/Al2O3/W structures

Fig. 5(a) shows a reduction in VSET when sequential switchingfrom HRS to LRS (up to 5 cycles in unipolar mode) is applied tothe same Al/Al2O3/Al-MIM device. Even though the same broadresistivity window is obtained (around 6 orders of magnitude) acontinuous reduction in VSET would limit the endurance of thesememory devices and their general reliability will also be compro-mised. We also notice unipolar behavior, which is obtained byapplying sequential voltage sweeps having the same polarity,through this device. Therefore, these memory devices can havetwo types of switching modes: unipolar and bipolar. Each switch-ing mode will depend mainly on the bias conditions and also, themagnitude of the compliance currents that will limit the total cur-rent after breakdown [17]. On the other hand, a reduced VSET couldbe related to the chemical reduction of Al2O3 and/or metal migra-tion from one of the MIM’s interfaces so that, after generation of aninitial conductive filament, incomplete dissolution of this filament(during consecutive I–V sweeps) would shorten the effective thick-ness of the oxide and therefore, a reduced VSET is needed for break-down. In addition, trapping of electrons at localized sites in theAl2O3 layer (as suggested by the Poole–Frenkel conduction mecha-nism) could possibly reduce the VSET progressively as well. Here,random thermal fluctuations will give trapped electrons enoughenergy to get out of its localized state and move to the conductionband, therefore, contributing to the total charge density that isneeded for breakdown. Whether an incomplete dissolution of aconductive filament or an increase in the density of electrons flow-ing in the Al2O3 BE direction (after electron trapping), the pro-gressive reduction in VSET compromises proper performance ofthese ReRAM devices and therefore, further study is needed inorder to minimize this effect.

The inset shows an idealized energy band diagram for this Al/Al2O3/Al system (at thermal equilibrium) and with the BE shownin bold letters. After alignment of the Fermi level, symmetric I–Vcharacteristics should be expected.



Fig. 5. (a) Unipolar resistive switching of Al/Al2O3/Al structures where VSET is progressively reduced for continuous I–V cycles. The IOFF/ION ratio for this structure is also high,around 106. CC = 100 lA. The inset shows the idealized energy band diagram for this MIM structure. (b) C–V characteristics of Al/Al2O3/W structures. The relative dielectricconstant for Al2O3 is extracted from these curves and is around eR � 6.5 (at 100 kHz). The inset shows the idealized energy band diagram for this MIM structure.


On the other hand, Fig. 5(b) shows the average gate capacitance(taken at Vg = 0 V, measured from 1 kHz – 3 MHz) after measuringseveral Al/Al2O3/W-MIM devices. From these data, the dielectricconstant relative to Al2O3 can be extracted and is around eR � 6.5(at 100 kHz), which suggest a chemical reduction of Al2O3 to AlxOywhen the oxide makes physical contact with the top aluminumelectrode. It is important to notice that this value for eR is obtainedby considering uniform oxide thickness (Al2O3 = 10 nm) and verylow surface roughness in the electrodes (especially at the surfaceof the BE), but this is not the case as will be shown in the next para-graphs. The inset shows an idealized energy band diagram for thisAl/Al2O3/W system (at thermal equilibrium) and with the BEshown in bold letters. After alignment of the Fermi level, asymmet-ric I–V characteristics should be expected. We notice that by usingtungsten as BE, a reduced potential difference is needed in order toinduce the same density of electrons flowing in the TE BE direc-tion (because of the higher work function for tungsten as comparedto aluminum). Therefore, it is desirable to use tungsten as BEbecause of its highly inert properties (thus increasing the thermo-dynamic stability of an Al2O3/W interface after thermal processing)and the low power consumption expected from these MIM devices(although this could be achieved by using other BE and TE materi-als having a large difference in their work functions).

3.4. Surface roughness of the bottom electrode after atomic-forcemicroscopy

In spite of the smoother I–V characteristics obtained whenusing aluminum as BE, or the expected dual advantage of usingtungsten as BE in MIM devices, the roughness presented at the sur-face of both electrodes makes them inadequate when compared tothe physical thickness of Al2O3. Fig. 6 shows the surface morphol-ogy of the aluminum and tungsten layers when used as bottom

Fig. 6. Surface roughness measured from the bottom electrodes (a) Aluminum and (b)thickness of Al2O3 in the MIM devices. Area is 10 lm � 10 lm.


electrode in the MIM structures (both metals were evaporated inultra-high vacuum conditions at an evaporation rate of 1 Å/s).The average surface roughness for the aluminum and tungsten lay-ers is 7.5 and 5.8 nm respectively (areas of 10 lm � 10 lm, 30 dif-ferent spots for each sample). A high level of surface roughnesscould be related to the formation of stacking faults after evapora-tion of these metals because of unstable sub-oxide formation ontheir surfaces after a rapid exposure to oxygen sources [18,19].Since the surface roughness for both metals is close to the physicalthickness of Al2O3, it is important to reduce this severe surfaceroughness (in both BE and TE) in order to obtain reproducible cyc-lic I–V characteristics. We think that statistical variations in VFORM,VSET and VRESET (after measuring several MIM structures and com-paring the results of only the first I–V cycles) are due to variationsin the effective physical thickness of the oxide given that both TE/Al2O3 and Al2O3/BE interfaces present a high degree of roughness.Besides the surface roughness of these metals, understandingchemically/physically-driven surface effects on the resistiveswitching of MIM devices is essential for proper performance ofthese structures [20].

In an effort to minimize the surface roughness of the BE, we per-formed aluminum evaporation experiments using different evapo-ration rates for this metal (while keeping a lower vacuum level,�2 � 10�7 Torr). Fig. 7(a–d) shows the typical surface morphologyfor aluminum after using the following evaporation rates: (a) 1 Å/s,(b) 2 Å/s, (c) 5 Å/s and (d) 10 Å/s. In these AFM images, the samevertical scale of 38 nm was used for proper comparison amongall samples. The corresponding surface roughness data (areas of10 lm � 10 lm, 30 different spots for each sample) for all caseswere: (a) 5.24 nm, (b) 2.87 nm, (c) 2.78 nm and (d) 2.33 nm. It isinteresting to note a trend of reduction in surface roughness whenhigher aluminum evaporation rates are used. Of course, the fastestevaporation rate for aluminum (10 Å/s) requires the minimum

Tungsten. Both metals show large surface roughness, quite close to the physical



Fig. 7. Surface roughness measured from four bottom electrodes made of Aluminum and deposited by e-beam evaporation at (a) 1 Å/s, (b) 2 Å/s, (c) 5 Å/s, and (d) 10 Å/s. Atrend of reduction in surface roughness is obtained for higher aluminum evaporation rates. Each area is 10lm � 10lm.


evaporation time. Therefore, exposure of the continuously increas-ing metal layers to any heat source within the evaporation cham-ber (self-heating during power delivery to the aluminum pellets) isminimized and thus, a final metal surface with smoother morpho-logical and structural characteristics is obtained. Reducing theroughness of the metal surface can be additionally promoted byslowly exposing the final surface to a controlled inert atmosphere.These last experiments are still underway and all results will bepublished elsewhere.

On the other hand, the corresponding I–V characteristics ofcomplete Al/Al2O3/Al structures (where the aluminum BE is depos-ited with the former evaporation rates), show highly-dependentconduction mechanisms and electric fields intensities E (beforebreakdown) to these surface roughness characteristics. Fig. 8shows the J–E characteristics of these Al/Al2O3/Al-MIM structuresin the unipolar switching mode. We only show the first sweepcycle for clear comparison of the conduction characteristics in allsamples. Additionally, we have included the J–E characteristics ofthe Al/Al2O3/nSi-MIS structures (see Fig. 2) into the same plot inorder to compare the electrical behavior of the MIM devices witha sample having minimum roughness in the BE (formed by the sil-icon surface in accumulation). We notice that the MIS structurepresents higher current densities after an electric field of around

Fig. 8. Unipolar resistive switching of Al/Al2O3/Al structures (first cycle) where theelectric field for hard breakdown is progressively increased and correlated to aminimum surface roughness of the aluminum-based bottom electrode. Theminimum surface roughness for the aluminum electrode is found when this metalis deposited at higher evaporation rates. The J–E characteristics of a MIS sample (Al/Al2O3/nSi) have been also included for comparison.


E � 3 MV/cm. This is expected since for the MIS device, a thinnerAl2O3 = 6 nm was used. Also, the electric field for breakdown forthe MIS device is Ebd = 7.4 MV/cm, an outstanding value forAl2O3 and which is related to the quality offered by its depositiontechnique. Interestingly, this Ebd is similar to the MIM structurehaving the lowest surface roughness, thus giving us an idea aboutthe importance of having minimum surface roughness on the BE.

Now, for the slower (a)–(b) evaporation rates, the J–E curvesshow the highest gate current densities while the breakdownevents occur at very low electric fields (using 10 nm as the physicalthickness for Al2O3). Additionally, single conduction mechanismsare noticed just before breakdown while their poor electricalbehavior is in correlation to the high surface roughness of theirBE’s. In contrast, the faster (c)–(d) aluminum evaporation ratesshow the lowest gate leakage current densities while the break-down events occur at higher electric fields. Also, it is clear thatmore than one single conduction mechanism is present in thosedevices and these characteristics could be correlated to the lowersurface roughness of their BE’s. However, we must consider thateven though the difference in surface roughness for the (b) and(c) samples is minimal, their J–E characteristics are dramaticallydifferent so that a direct correlation of low surface roughness toenhanced I–V characteristics, although true, is still incomplete.With this in mind, it is also important to consider any chemicalchanges occurring at these interfaces since lower or higher interfa-cial roughness would promote different chemical reactions withAl2O3, thus creating interfacial layers (IL) of different characteris-tics. Nevertheless, these data confirms the importance of havinglow surface roughness on the aluminum electrodes when Al2O3-based MIM devices are used. From the observed experimental data,samples having the lowest surface roughness on the BE showed ahigher number of I–V cycles whereas a reduced number of cycleswere obtained for MIM devices with higher surface roughness. Thiseffect was more pronounced when the physical thickness of Al2O3

was increased to 20 nm, thus minimizing the effect of roughnesson the BE. By doubling the Al2O3 thickness, most variations inVFORM, VSET and VRESET are reduced notably as well, because athicker oxide screens-out the surface roughness of both the BEand TE while the power consumption during memory operationis increased (current/voltage levels are both increased for the setand reset operations, data not shown).

Finally, even though the surface roughness of aluminum andtungsten is comparable with the Al2O3 thickness and all together,compromises the reliability of these devices, there are some prom-ising features related to their fabrication process. That is, these




samples have shown the memristance effect while using low tem-perature processing and standard materials used in BEOL. By opti-mizing the physical and processing parameters of these structures,vertical integration of dense memory arrays using MIM structurescould be implemented at BEOL processing in order to obtain den-ser, smarter and highly efficient integrated circuits.

4. Conclusions

MIM devices based on Al/Al2O3/Al and Al/Al2O3/W structureswere fabricated on glass using low thermal processing and theirelectrical and morphological characteristics were obtained andcompared when using highly reactive or inert metals as the bottomelectrode. The memristor effect has been observed in the bipolarand unipolar switching modes for both structures and those modeswere limited by the amount of electrons tunneling through thedevices (by setting a fixed current compliance CC). The expectedrelationship of VFORM > VSET > VRESET was found while the IOFF/ION

ratio is around 4-6 orders of magnitude. By measuring severalresistive switching cycles in Al/Al2O3/Al structures, VSET is progres-sively reduced and that is related to an oxidation-reduction mech-anism that decreases the effective oxide thickness of the MIMdevice. From C–V measurements done on Al/Al2O3/W structures,a relatively low dielectric constant for Al2O3 was found (eR � 6.5at 100 kHz), and which suggest a chemical reduction of Al2O3 toAlxOy when the oxide makes physical contact with any of the alu-minum electrodes. Also, the surface roughness of aluminum andtungsten is comparable with the Al2O3 thickness and all together,compromises the electrical and reliability characteristics of thesedevices. Nevertheless, a low surface roughness of the bottom elec-trode plays an important role for enhancing the electrical charac-teristics of these MIM devices by obtaining lower gate currentdensities and higher electric fields for breakdown during cyclic


voltage sweeps. Finally, because of their low thermal budget, theseMIM structures have the potential (after optimizing their electricalresponse) to be vertically integrated into a CMOS-based BEOL pro-cessing in order to obtain denser, smarter and highly efficient inte-grated circuits.

Acknowledgments

This work was fully supported by the National Council of Sci-ence and Technology (CONACyT-Mexico).

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influence of the surface roughness of the bottom electrode on the resistive-switching...

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