Impedance Spectroscopy of novel Access Devices based on ...benji20.free.fr/IBM_MIEC/IBM_Report.pdfFirst, ince it is admited that current in MIEC materials is both due to electrons

Master Nanotech

Phelma, Grenoble INP

Academic year: 2010-2011

IBM Research - Almaden

650 Harry Road

San Jose, CA 95120

Impedance Spectroscopy of novel Access Devicesbased on Mixed Ionic Electronic Conduction

Author:

Benjamin Meunier

[email protected]

Supervisors:

Georey Burr

[email protected]

April-August 2012

mailto:[email protected]

mailto:[email protected]

1

Contents

Introduction 4

1 Global presentation of the master thesis 6

1.1 IBM Almaden Research Center . . . . . . . . . . . . . . . . . 61.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2.1 Storage-Class Memory (SCM) . . . . . . . . . . . . . . 71.2.2 Flash limitation . . . . . . . . . . . . . . . . . . . . . . 71.2.3 3D Multi-layer SCM . . . . . . . . . . . . . . . . . . . 8

1.3 State-of-Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.3.1 General principle . . . . . . . . . . . . . . . . . . . . 91.3.2 Mathematical model . . . . . . . . . . . . . . . . . . . 101.3.3 Previous results from IBM . . . . . . . . . . . . . . . . 12

2 Description of the experiments 16

2.1 IV characteristics of MIEC . . . . . . . . . . . . . . . . . . . . 162.1.1 Conductive AFM measurement . . . . . . . . . . . . . 162.1.2 Description of the results . . . . . . . . . . . . . . . . 18

2.2 Impedance spectroscopy . . . . . . . . . . . . . . . . . . . . . 222.2.1 Principle of IS . . . . . . . . . . . . . . . . . . . . . . 222.2.2 Limitation . . . . . . . . . . . . . . . . . . . . . . . . . 242.2.3 Sample sizing . . . . . . . . . . . . . . . . . . . . . . . 25

3 IS Results and discussion 32

3.1 Theoritical model for MIEC Impedance . . . . . . . . . . . . 323.1.1 Band Diagram-based model (model 1) . . . . . . . . . 323.1.2 Electrolyte like model (model 2) . . . . . . . . . . . . 333.1.3 First MIEC IS and tting . . . . . . . . . . . . . . . . 36

3.2 Started experiments . . . . . . . . . . . . . . . . . . . . . . . 383.2.1 Eect of the Bias . . . . . . . . . . . . . . . . . . . . . 383.2.2 Eect of the thickness . . . . . . . . . . . . . . . . . . 40

3.3 Next steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Conclusion 44

2

Appendix 47

A.1 Mathematical complement . . . . . . . . . . . . . . . . . . . . 48A.1.1 Defect model p<<n<<N and N uniform . . . . . . . . 48A.1.2 Defect model p<<n=N . . . . . . . . . . . . . . . . . 50A.1.3 Relaxation Process . . . . . . . . . . . . . . . . . . . . 52

3

Introduction

Memory eect may be nd everywhere even on the cup lid of your favoritesoda. But its not enough to design a memory. Indeed a memory requiresa suitale environemt to be ecient. One part of this environement is theaccess device which is a small device able to control when the memory cellis used and when is not.

This master's thesis was focuson these access device studied byIBM in order to be compatiblewith jointly developped novel Non-volatile memories. The workingprinciple of these novel devices isbased on Mixed Ionic ElectronicConduction. This physical phe-nomenon involving both ionic andelectronic conduction is further ex-plain in the rst chapter.

The IV characteristics have still been well explored by the IBM researchteam I was working with. Since, this device is intended to electronic appli-cation, its impedance must be known as well. so, my involvement in thisproject will start from the very begining of the Impedance measurement. Inthis report, I will rst show the dierent steps I perform to characterize themeasurement apparatus and to establish a measurement process. Then, Iwill present the results I achieved and exploit them according to dierenttheoritical models.

4

5

Chapter 1

Global presentation of the

master thesis

1.1 IBM Almaden Research Center

Located in the middle of the hills of Santa Teresa Country Park in the southof San Jose, the capital of the silicon valley, the IBM Almaden ResearchCenter (ARC) is the dream spot for a master thesis in microtechnoly engi-neering domaines. I took advantage of both the proximity with the huge ITcompanies and a natural living environment suitable for an ecient work.Moreover, IBM is the third largest company in the area and within its wallsare working great names of the science research of the last decades. The IBMARC is mainly focused on the computer sciences from the fundmental aspecttill the production. There is lot of research regarding the storage devices ;my tutor, Georey Burr and his team are precisely working on non-volatilmemories.

6

CHAPTER 1. GLOBAL PRESENTATION OF THE MASTER THESIS

1.2 Motivation

1.2.1 Storage-Class Memory (SCM)

Each day, in every place, numeric data are generated and must be stored.Large amount of those data are stored on "archive" devices such as Opticaldisk, Hard disk or Magnetic. Even more data are also stored on so-calledStorage-Class Memories (SCM). They combines the advantage of both solid-state memories (high performance and robustness) and conventional hard-disk magnetic storage (archival capabilities and low cost).[1] Nowadays, theonly suitable technology is the Flash Memory [2]. Flash memories are widelyused as storage device (SD cards, USB drives, SSD ...) and their market isexpected to surpass $29 billion in 2014 [3]. Even if their price is decreasing,years after years (see Figure 1.1 below), it is still far from the HDD but it isnot the only limitation.

Figure 1.1: NAND Unlikely to Match HDD $/GB. sourc : Objective-Analysis(Understanding the NAND Market)

1.2.2 Flash limitation

Indeed, Flash memories have two main technical limitations. First, it shouldbe dicult to keep shrinking the size of the ash cells.[4, 5] It seems dicultto keep following the Moore's Law especially because of the transistors ordiodes used as Access Devices (AC)[6]. In order to increase the data densityand hence reduce the size of the memory array, Multi-level cell has beendevelopped. This cell is able to store several bits of information for dierentpotential level. The drawback of this technic is drastic loss of endurance asillustrated on the gure 1.2 below [7].

The Scaling Barrier and the Endurance loss are two of the reasons whymany dierent novel Non-volatile memories (NVM) are studied (classica-tion of the dierent technologies given in gure A.1 from ITRS 2010). Phase-

7


Figure 1.2: Increase of the memory cell capacity reduce signicantly theendurance

Change Memory1 and Resistive RAM are both been studied by the IBMteam[?]

1.2.3 3D Multi-layer SCM

Moreover, these novel NVM could allow dense 3D multilayer architectures.In this architecture, several memory cells are stack on several dierent layers(see the gure 1.3) which signicantly increase the density of data. To controlthe access to only one memory cell, novel access devices must be suitablewith the 3D architecture involving the following requirements :

High current for the ON state (up to 100µA)

Low leakage for the OFF state (less than 100pA)

Back-End-Of-Line (BEOL) compatible ( 400 °C processing)

Figure 1.3: 3D multi-layer architecture : solution for huge density SCM

1PCM exploits the resistance contrast between the amorphous and crystalline states.[8]

8


Unfortunatly, conventionall silicon diodes do not meet all these require-ments. It has been demonstrated that Cu-containing Mixed Ionic ElectronicConduction (MIEC) materials allow an ultra high current density as well asan ultra-low leakage (< 10pA) and their fabrication process is BEOL-friendly[9, 10].

MIEC-based novel access device seems to be a good candidate of 3Dcompatible AD. The successful development of this device could be a realboon for the novel Non-volatile Memories and the future of the Storage-ClassMemories. However, their behavior is not still perfectly understood especialythe impedance of such device has not been exploring.

1.3 State-of-Art

1.3.1 General principle

As its name implies, conduction in Mixed Ionic Electronic Conduction ma-terials is both due to electrons and ions. More precisely, in such materialelectronic current is gated by ionic motion. The resulting behavior is a dou-ble diode-like conduction.

Figure 1.4: Metal-MIEC-Metal

MIEC may be exhibited in some spe-cic nonstochiometric solids, for exam-ple α−Ag2+δS wich conduct both Ag+

ions and electrons [11]. The composi-tion of the MIEC material used by IBMis kept condential. The only publicinformation is that copper ions are in-volved. Cu ions and Cu vacancies actas acceptors and donors on the MIECmaterial which may be either n-type orp-type semiconductor. Once metal isadded on two sides, a MSM structureis obtained.

On the gure 1.5 the behavior ofsuch device is described on experimen-tal IV-characteristcs (see 2.1 for moredetails on the measurement process).

9


Figure 1.5: Description of the dierent steps of IV-characteristic :1 OFF State : The current ow is due to the tunnel electrons ow passingthrough the sharp potential barrier.2 ON State : Barrier to hole injection is reduced. The current growsexponentially.3 Saturation : The current grows linearly due to space charge eects.

1.3.2 Mathematical model

Charge Distribution

First, ince it is admited that current in MIEC materials is both due toelectrons and single ionized ions, the two density of current may be exprssedby :

je =σeq∇µe (1.1)

jion = −σionq∇µion (1.2)

where σ is the electrical conductivity dened by σ = zqν[c] with q theelectronic charge, ν the mobility and [c] the particle concentration. µ isthe elctrochimical potential, that is to say it is the sum of the chimical andelectrical potential :

µ = µ+ zqΦ (1.3)

10


For low particle concentration, the chimical potential follows the Boltz-man's approximation µ = µ0 + kBT ln[ p

p0]. Since we admit that conduction

only take place along one direction into the material and E = ∂Φ∂x , the two

following expression of current density are obtained :

je = νekBT∂n

∂x− qνen

∂Φ

∂x(1.4)

jion = −νionkBT∂N

∂x− qνionN

∂Φ

∂x(1.5)

with n, N the particle concentration of electrons and ions

The Poisson's equation may be involved as well :

∂E

∂x=q

ε[N − n] (1.6)

The two sets of equation (1.4 + 1.6) and (1.5 + 1.6) make a system ofrst order non-linear dierential equation which describe the charge distri-bution along x. The solution may be computed, but three constraints mustbe dened (Boundary conditions, Mass action law ...) in order to obtain asingle solution[12].

IV behavior

In this part we will see two models used to determine the characteristic equa-tion of MIEC material and discuss of the result. The detailed calculationscan be found in appendix A.1.

Defect model p<<n<<N and N uniform In this model, N, the ionparticle concentration is concidered uniform and huge compared to n, theelectron particle concentration. We dene the emf Vth.

n(x = L)

n(x = 0)= e−qβVth (1.7)

We assume the electrodes are Ion Blocking electrodes. That means thatCu+ cannot pass through either the top nor the bottom electrode.

1

L

∫ L

0n(x)dx = nav (1.8)

Under this conditions the I-V relation is predicted by [13] :

je = −2navνeβL

tanh(1

2βqV ) (1.9)

11


Defect model p<<n=N In this model, the previous asumptions are stillright as well as the equation 1.8. The only dierence since n = N is the valueof the constant ration between the charge concentration at each side :

n(x = L)

n(x = 0)= e−

12qβVth (1.10)

Hence the I-V relation become :

je = −4navνeβL

tanh(1

4βqV ) (1.11)

On the following graph the two I-V relations are ploted for a 30 nm thinsample and at 300K. They will be compare latter with experimental results.

Figure 1.6: MATLAB graph of IV characteristic according to the model 1(blue) and the model 2 (green). The Y-axis represent the current normalizedby the electron mobility and the average electron concentration.

1.3.3 Previous results from IBM

Only 12 papers focus on MIEC-based access device have been published andamong those, 4 are from IBM.

100% yield

One encouraging result has been presenting in VLSI 2012 by Georey W.Burr[14]. This paper presents measurements made on large array MIECdevices (512x1024) . A yield of 99,955% has been achieved on optimizeddevices after BEOL processing conditions. The yield is estimated regarding

12


the voltage margin which corresponds to the dierence between the positiveand the negative value of the volatge for a given current. On the two followinggraph array of optimized can be compared with unoptimized devices. Theyield is signicantly lower, several devices are leaky and a pronouced "edgeeect" is exhibited.

Figure 1.7: A. 512x2014 array of with no edge eect and 100% yield. B.Unoptimized process. The nonresponsive bitlines seen here are not relatedto the MIEC process

Figure 1.8: Within this same 512x1024 array, there were no leaky devices ;100% of the array showed Vm > 1.1V . 99.955% of MIEC ADs had voltagemargins Vm at 10nA within ±150mV of median of 1.36V .

13


Endurance

A paper of Rohit Shenoy for VLSI 2011[15] sutied the endurance of suchdevices. This endurance is limited by the transition from low-leakage to ahigh-leakage which occurs very abruptly after a certain amount of cycles(1.9). It has been shown that at low-current (< 10µA), these favorable ADcharacteristics persist for 1010 switching cycles. It is also exhibited thefact that higher is the current lower is the endurance (1.10).

Figure 1.9: Abrupt change from low- to high-leakage occurs after many cycles

Figure 1.10: MIEC-based AD endurance depends on current, but is inde-pendent of BEC CD, despite the nearly 3-fold change in current density

14


15

Chapter 2

Description of the experiments

2.1 IV characteristics of MIEC

2.1.1 Conductive AFM measurement

To realize the IV measurement an AFM has been used. The goal, here, isnot to obtain a topography of our sample but use the very sharp AFM tipas a testing probe make contact with the device. Indeed, the MIEC devicesare only 100 nm diameter as it is shown on the picture above

Figure 2.1: TEM picture of one device

The interface used to run the AFM is provided by Igor. Moreover, Igorwas used to anlyze large array of data. Indeed, since we are interested in theyield of such device, we need to have an ecient tool able to compute andplot all the collected data.

16

CHAPTER 2. DESCRIPTION OF THE EXPERIMENTS

Figure 2.2: Description of the AFM

17


2.1.2 Description of the results

Qualitative Analysis

The measurment are achieved and the resulting IV-curves plotted. The gure2.3 shows two typical results obtained on two dierent sample. The currentis represented on a logarithmic scale which that means that the linear partscorrespond to the exponential behavior predicted by the equation 1.9 and1.11. Moreover, plateaux of current are also exhibited for higher voltagebut the current never 0A because of leakage current which are taking intoaccount by the calculation models.

Figure 2.3: IV-characteristics of two sample of the same substrat.

18


Nevertheless, it must be noticed that the leakage current is smaller than10pA as required. The two sample composition is the same, they have thesame substrate and the materials of their electrodes are the same too. Theonly dierence between them is the size of the bottom electrode. The bot-tom electrode of the rst sample is much smaller then the top one andthe consequence is an asymmetric IV-characteristic. Inversely, the bottomelectrode size of the second sample is close to the top electrode and theIV-characteristic are much more symmetric. It can be assumed that thesymmetry of the IV-characteristics is directly linked to the symmetry of thedevice.

Yield and Voltage Margin

MIEC ADs are developed for large arrays applications. That means we arenot only interesting by having one working device but by having as manydevices as possible with the wanted propeties. The most signicant propertyfor AD is the Voltage margin, Vm. As it is represented on the gure 2.4the voltage margin corresponds to the dierence between the positive valueand the negative value of the voltage for a given current. We concider thatVm = 1.5V is sucient for PCM and RRAM (see crosspoint memories 1.2.3).

Figure 2.4: Voltage margin of about 1.4V

On the last graph the IV-characteristic of 14 devices have been ploted.Each of these devices has a dierent Voltage margin. Actualy these 14 de-vices correspond to a line of the tested area (see on the left gure 2.6). Ifwe know concider the whole scan area, we can create a map of the dierentvoltage margin as shown on the right on graph 2.6. On this map each deviceis represented by a color corresponding to its voltage margin. 1.4V is con-cidering as the lowest acceptable value and the yield is calculated regarding

19


the device with a larger voltage margin. The same process has been appliedin the paper [14] exhibiting a 100% yield and detailed in part 1.3.3. Thedevices in red are too leaky to be used and the ones in purple don't work atall.

Figure 2.5: On the left: AFM scan of the tested area, each bright grey dot isa MIEC device. On the left: map of the same area, the devices are replacedby their voltage margin.

An other way to present these results is to plot the cumulative distri-bution of the voltage margin all over the tested area. On the graph ?? wecan see that most of the devices (92% of them) have a voltage margin arecontained in a window of ±0.18V around 1.35V . This graph also exhibitsthe fact that some devices are very leaky (Vm = 0.48 for the lowest value).

All these measurements are achieved on several sample with dierentfabrication process. The aim is only to test the dierent samples. Sincethe process is kept condential there is no real feedback on these results.Impedance measurement has never been achieved of MIEC material and itcold be a good way to characterize the devices and to try to understsandmore about this material.

20


Figure 2.6: Cumulative distribution of the voltage margin

21


2.2 Impedance spectroscopy

2.2.1 Principle of IS

Impedance spectroscpy is a powerful method of chatracterization many ofthe electrical properties of materials and their interface[16]. In the case ofMIEC study, IS could be both used to determine the general behavior of thedevice for high frequencies and to better understand its working principle.

Figure 2.7: Calculation process of FRA for impedance measurement

There are dierent computational methods for Potential Electrochemi-cal Impedance Spectroscopy (meaning we are working at constant potential)among which we nd the so-call FRA for Frequency Response Analysis. Thismethod is the one that will be used for the measurements and its computa-tional process is described on the graph 2.7. First, a periodic perturbation isgenerated as well as two reference signals which will be used to calculate thereal part and imaginary part. This perturbation is applied on the sample andthe response is measured. This response is composed by three components,the transfert function of the sample accompanied by some harmonics due tothe non-linearity of the systeme and a noise signal.

The measurements are realized in single-sine mode. In this mode, theperturbation amplitude must be small enough to operate in linear region,.That means that the harmonics are neglected. since we consider the noisevery small compare to the main signal, it is admited that the response signalis only composed by the transfert function of the sample.

22


Figure 2.8: Single-sine mode : The perturbation is a purely sinusoidal signalof small amplitude in the sort that harmonics of the response can be neglected

This transfert function is compared to the two references signal andonce we go in the frequency domain, the real and the imaginary part ofthe impedance, function of the frequency can directly be read. They canalso directly be ploted, but the choice I made is to use Bode and Phase di-agram, more convenient to analyze the eect of each "electric component".The following picture shows am example of Bode Diagram.

Figure 2.9: Example of Impedance spectroscopy result and analysis. Thepurple curve is the Bode Diagram and the green curve is the Phase diagram.We can, without any analysis tool, read a lot of information about the speciccircuit.

23


2.2.2 Limitation

However, one of the most signicant limitation is the existence of equivalentcircuit. Indeed if we concider the two following electric circuits, they havethe same impedance for all the frequencies. Hence ther bode diagram ofcircuit A () will be the same than the one of circuit B.

Figure 2.10: Two circuit having the same impedance whatever the appliedfrequency is.

The main issue is that two dierent electrical behavior also means twodierent physical phenomena. One solution is to have a physical model inwhich we can have faith. A technical solution now, is to test dierent size ofsample and under dierent conditions in order to select one circuit. Actually,we can see a good analogy betwween the Impedance spectroscopy and themight of Münchausen baron1. Long story short, Impedance characterizationis a purely circularity process2. Indeed in our case, we need to assume themodel is true to carry out measurements and we need results to assure theavailability of the model.

1Baron who allegedly pulled himself and the horse on which he was sitting out of aswamp by his own hair

2one of the ve tropes of Sextus Empiricus about the ability of the truth

24


2.2.3 Sample sizing

First IS : a size issue

The rst experiment is the impedance measurement of the sample used forIV-measurement. The same AFM is used and the BioLogic SP-300 is con-nected to computer and on both side of the sample ; the voltage will beapplied on the top electrode and the bulk is connected to the ground. Theapparatus requires EC-Lab instead of Igor.

Before running the measurement, several parameters must be set. First,the Bias voltage ( VBIAS in the gure 2.8) must chosen. It is selected inorder to operate in linear region. The amplitude of the perturbation mustbe set as well. For the same reason of linearity, it shouldn't be to large, butit shouldn't be to small neither to get relevant data. Into linear reagion, itis set to 10 mV. Then, other parameter may be chosen like, the number ofpoint per decade or the frequency range. This one is limited to 2 MHz in highfrequency because of the apparatus. Thus are run the rst IS measurementson several devices for a bias of -0.2 V.

Figure 2.11: Bode diagram made on AFM

Figure 2.12: Phase diagram made on AFM

25


Measurement are realized thanks to the EC-Lab software but they areplot on Igor which he is more convenient. The only requirement is to writean Igor process able to read and understand data from IS measurement. SoI did.

If we look at the Bode and the Phase Diagram, the rst conclusion is theyare characteristic Impedance for purely capacitive circuits. Even if, at thistime, any model for MIEC impedance was involved, at least, a rst orderbehavior was excpected. On the gure 2.13, the MIEC and the AFM tipare represented. The gure A represent what is expected, a small resistancedue to the contact between the tip ans the sample in serie with the MIECImpedance. Nevertheless, regarding to the previous results, it seems thatthe MIEC impedance is very small compared to an unexpected capacitance[17]. This parasitic capacitance should be located between the tip and thetop contact of the sample as shown on the gure B.

Figure 2.13: Representation of the sample and AFM tip during IS measure-ment. The gure A shows what is expected and the gure B what mayreally be measured

In order to gure out this issue we need to neglect the tip capacitancecompared to MIEC impedance. The easiest way to increase the ratio betweenthe two impedance is to increase the one of MIEC. In order to do so, newsamples have been redesign with a huge area.

26


New Sample, New Apparatus

The new samples are designed with a scare top electrode of about 200µmlength. That means the area is thousand times larger than 110nm diameterdots. Regarding to the size of these new devices, the use of tha AFM is nolonger required. I can directly use a testing probe to measure the samples.This apparatus is described on the picture below.

Figure 2.14: Tool description for impedance spectroscopy

The optical microscop is connected to a television screen (no numericoutput). The holder is connected to vaccum pipes to maintain the sample.The potentiometer is connected on the red wire linked to the probe and tothe bulk of the sample via the magnet xed on the metal disk.

A set a four sample have been done. Two of them are MIEC sample(measurement in next chapter 3) and the two other are one resistance sampleand one capacitance sample. They were used to characterize the tool andto establish a relevant measurment process. The two next parts present thestudy of these two samples.

27


Resistance Sample

The sample is the simpliest one and it is the reason why he was studied rst.The gold top electrodes is directly deposited one a titanium nitride layer.

Figure 2.15: Resistance sample

The approach process is radically diferent to the AFM one. The onlyway to determine if the tip is touching the sample is opticaly. Indeed if onekeep moving the probe down, the tip of the probe seems to move verticaly,it means the probes is bending.

This method is not so precise, the load applied by the tip on the sample isnot exactly the same and there is no way to quantify. I also had to trade-owith the fact that we want to have a good contact betwwen the tip and thesample and the fact that we want to avoid demaging the MIEC bulk diggingto deep into the gold top electrode. There is no formal experiment to do so,just use and trainning.

Figure 2.16: Consequence of a toohigh current

Something else must be avoid,the explosion of the device. In-deed, in the right picture thedevices are totaly destroyed dueto a too high current passingthrough. In order to avoidthis kind of inconvenience a se-rial resistance is added. Itsvalue is about 50Ω for resistancesample and 100Ω for MIEC de-vices.

Last but not the least, I wanted to test the regularity of the results obtainwith such tool. Resistancce measurement are realized on several devices.The IV curve is plotted on EC-Lab (graph 2.17) and then tted by a lin-ear expression. The average value of the resistance is Rbulk = 20.6 ± 0.5Ω.Moreover, this resistance corresponds to the resistance of the silicon bulkplus titanium nitride, which will be a usefull data for the coming impedancemeasurements.

28


Figure 2.17: IV-characteristics of resistance samples

Capacitance sample

The capacitance sample is the second and last sample used to test the appa-ratus and establish a measurement process. This sample is composed by thegold top electrode and the TiN electrode separated by a 30nm thin siliconnitride layer. If the measurement are carried out, the following bode diagramare obtained :

Figure 2.18: Capacitance sample

Figure 2.19: Bode diagram and Phase diagram without parallel capacitance.We can see the −π

2 phase for high frequency but not the transition from 0.

It would be interesting to observe the cuto frequency which seems tobe lower than the frequency range. The Impedance is shiftef by adding alarge parallel resistance ( 50MΩ ), then the bode diagram are the one onthe gure 2.20.

29


Figure 2.20: The eect of adding a huge parallel resistance. The wholespectrum is shifted to higher frequency domain. The "complete" Bode andPhase diagram are visible and so are the cut-o frequency.

In order to test the reliability of the result, I measured the capacitanceof the same sample for dierent surface area. All the devices are supposed tohave the same size excepted the ones located on the border of the mask. Asit is shown on the gure 2.21 ve dierent sizes of device can be used. Theexact size of each of them is calculated thanks to an electronic microscopeand the ImageJ software.

Figure 2.21: Electronic micorscope view of the border of the sample. Fivedierent size of device can be involved

IS is done on each device and the graph of the capacitance versus thearea is represented on the graph 2.22. This graph exhibits the fact that for aconstant thickness, the capacitance is proportional to the area of the to elec-trodes. Nevertheless, we can also notice that the y-intercept, b, is not null.That means the total capacitance is composed a two parallel capacitance,

30


the rst one depends of the area and the second one, much smaller, doesn'tdepend of the area. It appears be the so-called parasitic capacitance saw inthe gure 2.13. Hence, we can estimate this capacitance Cparasit ≈ 20pF .

Figure 2.22: Capacitance versus the surface area. The linear behavior is theone expected excepted the non-null y-intercept b which may corresponds toa parallel capacitance independant of the area.

The capacitance can be expressed by C = εrε0t Area + Cparasit with

t = 30nm the thickness of the sample, ε0 the electric permittivity of free-space or vaccum permittivity. Since the slode is known, the relative permit-tivity, εr can be calculated. So I did and the estimated value for a siliconnitride capacitance is εr = 8.77. This value can be compared with otherexperimental values and DFT calculation in the table 2.1. If the fund valueis a bit larger than the ones in the litterature it must due to the fact thesample is not composed by pure silicon nitride.

Method Dielectric Constantb− Si3N4 exp. [18] 8.4 - 8.66b− Si3N4 DFT [19] 8.19

Table 2.1: Comparison of permittivity values

These two samples (resistance and capacitance) have been well studied.They were a good training to master the dierent tools. Moreover, they gaveusefull informations for the Impedance measurement of MIEC. We know thevalue of the parasitic capacitance and the resistance of the silicon bulk :

Cparasit ≈ 20pF (2.1)

Rbulk = 20.6Ω (2.2)

31

Chapter 3

IS Results and discussion

3.1 Theoritical model for MIEC Impedance

As it has been said in the part 2.2.2 dedicated to IS limitation, a theoriticalmodel is required to have relevant data. The two next parts will presenttwo models I used during the measurement. One is based on the banddiagram used to explain the IV-characteristics in part 1.3.1 and the other isan improvment based on an electrolytic model for aqueous solutions.

3.1.1 Band Diagram-based model (model 1)

To establish this prediction model of impedance for MIEC device, we startfrom the band diagram of a classic Metal-MIEC-Metal structure.

Figure 3.1: Schematic representation of Impedance coming from Band-diagram

32

CHAPTER 3. IS RESULTS AND DISCUSSION

The gure 3.1 is a schematic representation made in order to explainwhere components of the impedance are coming from. From top to bottom:

Ccont : Parallel-plate capacitance due to the two electrodes.

Rel : Resistance due to electron tunneling passing throuh the potentialbarrier between the dierent conduction bands.

Rion : Ionic resistance.

CDebye: proportionnal to 1λ corresponding to the zero-charge region.

Then, adding the serial resistance used to reduce the current, the electriccircuit 3.2 is obtained and its impedance can be expressed by:

ZMIEC = Rs+Rel(1 + 2jπf CDeb

2 Rion

1 + 2jπf CDeb2 (Rel +Rion) + 2jπfCcontRel(1 + 2jπfRion

CDeb2 )

Figure 3.2: Equivalent circuit for MIEC impedance

The same electrical circuit is given by A. Leshem [20] using a modelbased on the geometry and the charge distribution along the sample (analyticsolution of equations 1.4, 1.5 and 1.6).

3.1.2 Electrolyte like model (model 2)

The idea of this model is since conduction is due to both electronic and ioniccurrent, we can make an analogy between MIECand the conduction into anelectrolyte. In electrolyte model of impedance involves relaxation process.Hence, the rst step to describe this prediction model is to details what isrelaxation process composed by. When an electric eld E is applied to aninsulating material, the resulting polarization P may be divided into twoparts according to the time constant of the response (see graph 3.3).

a instantaneous polarization also called "high-frequency" polarization:P∞ = ε0(ε∞ − 1)E

a time-dependant polarization P ′(t) which converge to Ps :Ps = ε0(εs − 1)E

33


Figure 3.3: Polarization function of the time.

Once again, all the calculation details are reported in the Appendix A.1.3.To summarize, the polarization P may be expressed in the Laplacian domainby :

L(P ) =P∞

(s+ ω0)+

ω0Pss(s+ ω0)

(3.1)

For a constant electric eld, E, a relaxation process appears. The admittancecorresponding to this process is the following :

Y =A

l

L(j)

L(E)=A

l[sε0ε∞ + ε0(εs − ε∞)

sω0

s+ ω0] (3.2)

If we write C1 = Al ε0ε∞, C2 = A

l ε0(εs − ε∞) and RC2 = ω−10 = τ , the

previous admittance correspond to the following electrical circuit :

Figure 3.4: equivalent circuit for one relaxation process

At this point, an important usmption is made :

"[...] the origin of the frequency dependance of the conductiv-ity [is] due to relaxation of the ionic atmospher after the move-ment of the particules."[21]

Then Almond and West have demonstred that the conductivity in such ma-terial can be expressed as a conbination of several relaxation process plus anintrinsic resistance as depicted by the following graph :

34


Figure 3.5: equivalent circuit of an electrolyte with n relaxation process

Looking this circuit, we can notice the case n = 1 corresponds to themodel 1. Thus the cases n 6= 1 are just the model 1 plus n− 1 pole. n is thenumber of relaxation process, in the MIEC materials, we know that copperions are involved and so at least one relaxation process. Let's assume, thatCu+ and Cu2+ are both involved and n = 2 for the following.

35


3.1.3 First MIEC IS and tting

The MIEC samples used are the same than the capacitance sample exceptthat silicon nitride is replaced by MIEC material.

The graph 3.6 represents the phase and Bode diagram of the MIEC fordierent bias voltages (0.2V , −0.2V and 0.4V ) with a signal amplitude of10mV . The curves on this graph are not bold line but composed by a set of9 dierent devices on the same die. Once again, we can appreciate the wellreproducibility of the measurements.

Figure 3.6: Bode and phase diagram achieved at dierent bias points.

The general shape of these graph can be studied. It must be noticed thatthe curve at 0.2V doesn't match with the one at −0.2V , the IS behavior is

36


not as symmetric as the IV-characteristics. The gure 3.7 represent thebehavior of the model 1 (conclusion would have been the same with themodel 2) for f → 0 and f → ∞ considering that the capacitance acts likean open switch at low frequency and like an close one at high frequency.Thus at low frequency, the serial resistance plus the passive resistance of thedevice should be seen, and at high frequency, only the serial resistance sincethe device is short-circuited by the global capacitance. Measuring the valueof the impedance at 10Hz and 1MHz, the experimental data match withthis two equivalent circuit.

Figure 3.7: Equivalent circuit based on the model 1 for low and high fre-quencies.

To go further, only the data at −0.2V are considered (graph 3.8and theexperimental curve is tted by the simulated curves rst from the model1 (based on band diagram) in green and then by the model 2 (based onelectrolyte-like model for 2 relaxation process) in blue.

Figure 3.8: Phase diagram of MIEC sample. Red cross are for the experi-mental measurements, the green plain line for the tting with the model 1and the blue one for the model 2

37


The two prediction models seems well match with the experimental. Thisgraph is a typical graph among a large set of measurmement and all of themmatch as well. Obvioulsy the model 2 is the closest to the data because itis nothing more than the rst model plus one pole. Nevertheless, for thefollowing experiments the model will be used because the interpretation areeasier and the 2 relaxation process was an assumption made without realjustication elements.

3.2 Started experiments

In this part, several experiments and graphs are presented. However, regard-ing to actual understanding of MIEC no conlusion can be extract from theresulting plot. These graphs are hints for future work.

3.2.1 Eect of the Bias

The previous results show the IS for dierent voltage bias, but the IVcurves for the large devices has not been studied. The graph 3.9 is theIV-characteristic for the large sample. Since the area of the sample is largerthe current passing through is bigger too and the property of the MIECstudied before are no longer availbale especially the leakage current which isless signicant compared to the operating current. Nevertheless, in order tostudy the behavior of the sample on the whole voltage range (from −0.5Vto 0.5V ), IS are achieved for dierent voltage bias. The phase diagram 3.10summarize the results from the negative values in blue to the positive onesin red.

Figure 3.9: IV-characteristics for large sample

On the graph 3.10 we can see both a symmetric behavior on the left patrand a monotonous behavior on the right of the curve. To go further, sincethe theoritical model match pretty well with the experimental data, we cantry to estimate the value of each component for each voltage bias.

38


Figure 3.10: Phase diagram for a range of voltage bias from −0.5V to 0.5V .

I succed to extract the values of the dierent components thanks to t-ting algorithm (one from the software and one implement by myself usingdichotomy). The graphs above (3.11) shows the values of the componentsfrom the prediction model 1.

Figure 3.11: Components values

To remind, Ccont is theplate capacitance due to thetwo metal electrodes, Rel is therestance electronic or passive re-sistance (since it is the only re-sistance saw in DC mode) andCDebye is the capacitance cre-ate at the interface between theMIEC material and the metalcontact. The resulting plot arereproducible and promising butstill intriguing. First, the volt-age dependance of the capac-itance Ccont and its asymmet-ric behavior are surprising. Thetwo other components values arewell symmetrics but not aroundzero, but around −0.1V .

Unfortunately, the theoryand the understanding of this

39


material are not suciently developped to draw conclusion about these re-sults. Dierent thickness of samples are now used in order to have an ideaof how each component depends on the device size.

3.2.2 Eect of the thickness

Samples of three dierent thickness are realized, 60nm, 90nm and 120nm.The rst graph (3.12) shows the IV-characteristic for dierent thickness. Theresistance increases with the thickness as expected.

Figure 3.12: IV-characteristics for dierent sample thickness

Then, IS at −0.2V and 0.2V are carried out and the results are plot ongraph 3.14 and ??. Once again, the negative behavior can not be comparedto the positive one. The evolution of the pasitive resistance (low frequencyimpedance) respect the one despicted in the previous paragraph. Then,with the same process than previously, we can plot the value of the dierentelectrical components function of the frequency.

Several sets of capacitance and resistance evaluation have been made,they presented all more or less the same behavior (Two commented examplescan be seen in appendix ??).The conclusion of these two experiments isthat signicant further research will need to be performed before ImpedanceSpectroscopy can contribute to the theoretical understanding to the novel-diode MIEC material. In the next part, are presented some ideas for futureresearches.

40


Figure 3.13: Bode diagram and phase diagram at −0.2V

Figure 3.14: Bode diagram and phase diagram at 0.2V

41


3.3 Next steps

The rst inpedance spectroscopy results have generated much more questionthan answer. In order to gure out several complementary experiments maybe achieved.

The rst idea is to realize sample with both small device and large areadevices on the same substrate in order to test if the MIEC behavior is stillthe same. Indeed, regarding to the IV curves obtained on the large devices,we are not even sure that the MIEC is still as well as expected.

The last measurement shows a huge asymmetrism between the positiveand negative polarity. In order to investigate some sample must be testedwith the MIEC material etched as well as the top electrode (on the picture3.15). Indeed, the bottom electrode looks like an "innite" electrode com-pared to the one and it may be a reason of the asymetrism. The only issueof this new design is the process used to etch the MIEC also etch in thehorizontal direction.

Figure 3.15: Design for new samples, MIEC material has been in order tocorrespond with the top electrode area

SPICE simulations have been done during this internship. The electricalmodel was used with the value of the components at a cetrain voltage. Thenthe tranient behavior at the same voltage was simulated. The resulting graphis shown on the gure 3.16. If we compare this result with previous Voltage-time measurments[22] on the same gure, we can observe that the voltagejump has not been exhibited by the experimental data. One conclusion isthe prediction model is not available for very high frequencies. A way to testthis assumption would be to performe IS at higher frequency. Unfortunetlythe tool is able to run IS only till 2MHz.

Some other experiments have been started waiting a better understand-ing a the material to be usefull. The eect of the annealing can be testedas well as the eect of the electrode material as show on the graph 3.17 and3.18. Then, Gold or Copper diusion can be measrured by SIMS. Temper-ature dependance measurements can also be performed. The aim of theseexperiments is be able to have a general description of the future devices.

42


Figure 3.16: Transient voltage. Left : SPICE simulation based on the theo-ritical circuit. Right : Resulting curves[22]

Figure 3.17: Eect of the annealing on the IV-characteristics of the samesample

Figure 3.18: Eect of the TEC material on the IV-characteristics of the samesample

43

Conclusion

During this internship, I succeed to performe Impedance Spectroscopy onMIEC devices. The non-AFM IS setup has been established. New de-vices have been designed in order to perform IS properly. These large-areaMIEC samples allow measurement of sample impedance rather than "tip"impedance.

One of the most dignicant result is that Impedance Spectroscopy spectramatched to RC circuits from MIEC literature, at multiple bias conditions.To go further, tting procedure has been developed for circuit parameterextraction and even a extensions to existing circuit models has been intro-duced. This extension of the rst model shows connection between circuitmodels and relaxation processes

To conclude preliminary experiments have been performed vs. thickness,top-electrode material, and anneal conditions and give hints for future ex-periment. Despite all the work achieved during those six months numerousfuture experiments identied and initiated

44

Acknowledgment

I would like to rst thanks Kumar Virwani and Rohit Shenoy for their in-volvement from the very beginning to the last correction of the presentation.Of course, I thanks Luisa Bozanno for her kindness and energy she gaveus during this end-of-term internship. Nothing would have been possiblewithout her and the help of Carl Larson and Spike Narayan.

Thanks to Bulent N. Kurdi and Alvaro Padilla for their interst for mywork,Liz Fedde, Jane Frommer, Leslie Krupp, Larissa Clark for their help andadviceAmy Bowers, Mark Jurich, Bill Risks and all those who have contributed tothe achievement of these resultsfriendly atmosphere created by the other interns.

45

[1] G. Burr, B. Kurdi, J. Scott, C. Lam, K. Gopalakrishnan, and R. Shenoy,Overview of candidate device technologies for storage-class memory,IBM J. Res & Dev., vol. 54, pp. 449464, 2008.

[2] F. Masuoka, M. Asano, H. Iwahashi, T. Komuro, and S. Tanaka, A newash e2prom cell using triple polysilicon technology, Proceedings of theInternational Electron Devices Meeting, vol. 30, pp. 464467, 1984.

[3] Flash memory summit 2011, santa clara, ca, 2011.

[4] K. Kim and J. Choi, Future outlook of nand ash technology for 40nmnode and beyond, Proceedings of the IEEE Non-volatile Semiconductor

Memory Workshop, Monterey, CA, 2006, pp. 911, 2006.

[5] L. Grupp, J. Davis, and S. Swanson, The bleak future of nand ashmemory. unpubished, 2011.

[6] S. Lai, Non-volatile memories: A look into the future, in Intel, 2004.

[7] S. Lai, Flash meories: Success and challenges, IBM J. Res. & Dev.,vol. 52, pp. 529535, 2008.

[8] S. Raoux, G. Burr, M. Breitwisch, C. Rettner, Y.-C. Chen, R. Shelby,M. Salinga, D. Krebs, S.-H. Chen, H.-L. Lung, and C. Lam, Phase-change random access maemory : A scalable technoly, IBM J. Res &

Dev., vol. 52, pp. 465479, 2008.

[9] I. Yokota J. Phys. Soc. Japan, vol. 8, p. 595, 1953.

[10] I. Riess, Current-voltage relation and charge distribuition in mixedionic electronic solid conductors, J. Phys. Chem. Solids, vol. 47,pp. 129138, 1986.

[11] H. Schmalzried Solid State Reactions, p. 48, 1974.

[12] Y. Gil, O. Umurhan, Y. Tsur, and I. Riess, Recent calculations andmeasurments of i-v relations in simple devices based on thin nano versusthick layers of semiconductors with mobile acceptors or donors, SolidState Ionics, vol. 179, pp. 11871193, 2008.

46

[13] Y. Gil, O. Umurhan, Y. Tsur, and I. Riess, I-v relations in nano thinsemi-conductors with mobile acceptors or donors, Solid State Ionics,vol. 179, pp. 2432, 2008.

[14] G. Burr, K. Virwani, R. Shenoy, A. Padilla, M. BrightSky, M. Joseph,M. Lofaro, A. Kellock, R. King, A. Bowers, M. Jurich, C. Rettner,B. Jackson, D. Bethune, R. Shelby, T. Topuria, N. Arellano, P. Rice,B. Kurdi, and K. Gopalakrishnan, Large scale (512kbits) integra-tion of multilayer-ready acces-devices based on mixed-ionic-electronic-conduction (miec) at 100yield. unpublished, 2012.

[15] R. Shenoy, K. Gopalakrishnan, B. Jackson, K. Virwani, G. Burr, C. Ret-tner, A. Padilla, D. Bethune, R. Shelby, A. Kellock, M. Breitwisch,E. Joseph, R. Dasaka, R. King, K. Nguyen, A. Bowers, M. Jurich,T. Topuria, R. P.M., and B. Kurdi, Endurance and scaling trends ofnovel access-devices for multi-layer crosspoint-memory based on mixed-ionic-electronic-conduction (miec) materials. unpublished.

[16] E. Barsoukov and J. MacDonald, Impedance Spectroscopy : Theory,

Experiment, and Applications (second Edition). Wiley, 2005.

[17] L. Fumagelli, G. Ferrari, M. Sampietro, E. Martinez, J. Samitier, andG. Gomila, Nanoscale capacitance imagiing with attofarad resolutionusing ac current sensing atomic force microscopy, Nanotechnology,vol. 17, pp. 45814587, 2006.

[18] T. Goto and T. Hirai J. Mater. Sci, vol. 24, p. 821, 1989.

[19] D. Fischer, A. Curioni, S. Billeter, and W. Andreoni Phys. Rev. Lett.,vol. 92, p. 236405, 2004.

[20] A. Leshem, Nonlinear i-v relations and hysteresis in solid state de-vices based on oxide mixed-ionic-electronic conductors, Nanotechnol-

ogy, vol. 24, p. 254024, 2011.

[21] K. Jonsher, The "universal" dielectric response, Nature, vol. 267,p. 673, 1977.

[22] K. Gopalakrishnan, R. Shenoy, C. Rettner, K. Virwani, D. Bethune,R. Shelby, G. Burr, A. Kellock, R. King, K. Nguyen, A. Bowers, M. Ju-rich, B. Jackson, A. Friz, T. Topuria, P. Rice, and B. Kurdi, Highly-scalable novel acces device based on mixed ionic electronic conduction(miec) materials for high density phase change memory (pcm) arrays.unpublished, 2010.

47

Appendix

Figure A.1: Summarize chart of the dierent NVM technologies

A.1 Mathematical complement

A.1.1 Defect model p<<n<<N and N uniform

Current density given by 1.4 and 1.5 with N uniform :

je = νekBT∂n

∂x− qνen

∂Φ

∂x(A.1)

jion = −qνionN∂Φ

∂x(A.2)

48

APPENDIX

We consider the dissociation and recombinason equation as well as itsconsequence on the chimical potential :

X+ + e− X (A.3)ka

µion + µe = µX (A.4)

Applying the mass action law :

N · n = ka · aX We assume N constant

n0 = aX(0) · kaN

nL = aX(L) · kaN

We apply the Bolztman's approximation for the chemcal potential andwe dene the emf Vth (voltage for zero current passing through the material):

n0

nL=nX(0)

nX(L)= eβ(µ0X−µ

LX)

= eqβVth (A.5)

With β =1

kBT

Since ∂j∂x = 0 and N uniform E = −∂Φ

∂x , from A.1

n(x) =je

qνeE− kBT

qE

∂n

∂x

n(x) = A+Be−qβEx is the general solution of the di. eq.

n(x) =je

qνeE− kBT

qE[−BqβEe−qβEx] (A.6)

A = jeqνeE

and from x = 0 we know that B = n0−A so the single solutionof the dierential equation is :

n(x) = [n0 −je

qνeE]e−qβEx +

jeqνeE

(A.7)

Applying the previous equation for x = L :

je[1− e−qβEL] = qνeE(n0e−qβEL − nL)

je =qνeE[n0e

−qβEL − nL]

1− e−qβEL(A.8)

49

APPENDIX

From A.7 and A.8 and LE = Vth − V :

n(x) = n0[1− 1− e−qβVth1− e−qβ(Vth−V )

(1− e−qβ(Vth−V )x/L)] (A.9)

je = −qνen0Vth− V

L[1− e−qβVth−V ) − e−qβVth

1− e−qβ(Vth−V] (A.10)

If we concider the case of Ion blocking electrode :

1

L

∫ L

0n(x)dx = nav (A.11)

Vth − V → 0

The polynomial approximation of n(x) gives :

n(x) = n0[1− 1− e−qβVthqβ(Vth − V )

(qβ(Vth − V )x

L)] (A.12)

n is suposed to be linear along x which means n(x) = n0 + (nL − n0) xL .By analogy with the equation A.11:

n0(L+L

2(nL − 1)) = Lnav

n0 =2nav

1 + e−βqV(A.13)

By replacing this expression of n0 in the equation A.10, the nal expres-sion of je is:

je = −2navνeβL

tanh(1

2βqV ) (A.14)

A.1.2 Defect model p<<n=N

In this second model, N is no longer consider as constant :

je = νekBT∂n

∂x− qνen

∂Φ

∂x(A.15)

jion = −νionkBT∂N

∂x− qνionN

∂Φ

∂x(A.16)

and, in this model, n = N . Making 12(A.16-A.15) the following expression

is obtained :∂n

∂x= −β

2(jionνion− jeνe

) = n1 (A.17)

50

APPENDIX

The reaction A.3 is still vailable, by the same process than previously :

n2 = kaaxn0

nLe

12βqVth

The dierence in this equation compared to the previous model is the 12

in the exponientiel. Then making 12(A.16+A.15) the following expression is

obtained :∂Φ

∂x= − 1

2qn(jionνion

+jeνe

) =n2

n(A.18)

henceΦ(x)− Φ(0) =

n2

n1ln(

n

n0) (A.19)

Determination of n2n1

: The expression of the voltage is given by :

−qV = µe(L)− µe(0)

µe = µe − qV

µe = µ0e + kBT ln(

n

n0)

Combining with A.18:

−qV = kBT ln(nLn0

)− q(Φ(L)− Φ(0))

−qV = −1

2qVth +

1

2q2βVth

n2

n1

n2

n1=

1

βq(1− 2V

Vth) (A.20)

Replacing n2n1

by the previous expression in A.19

Φ(x) =1

βq(1− 2V

Vth)ln(

n

n0) (A.21)

The derivative is given by :

∂Φ

∂x=

1

βqn(1− 2V

Vth)∂n

∂x(A.22)

Coming back to the equation A.15:

je = [kBTνe −νeβ

(1− 2V

Vth)]∂n

∂x

je = −νekBT [2V

Vth]n0 − nL

L(A.23)

jion = 2νionkBT [1− V

Vth]n0 − nL

L(A.24)

Adding the A.18 to the previous equations :

je = −4navνeβL

tanh(1

4βqV ) (A.25)

51

APPENDIX

A.1.3 Relaxation Process

The current is expressed by :

i =dD

dt(A.26)

Where D is the electric displacement andD = ε0E+P with P the polirazition:

P = P∞ + P ′(t) (A.27)

The polarization is composed by a instantaneous polarization term P∞ =(ε∞ − 1)ε0E and a time-dependant term P ′(t) (see graph 3.3). The limit ofP ′(t) for a long time polarization is called Ps = (εs − 1)ε0E. If we considerP ′(t) is a rst order kinetic then :

τdP ′(t)

dt= Ps − P (A.28)

In the Laplace domain :

L(P ′(t)) =ω0

s[1

sPs − L(P )

L(P ) =1

sP∞ + L(P ′(t))

L(P ) =P∞

(s+ ω0+

ω0Pss(s+ ω0

(A.29)

If the current expression A.26 is derived in the Laplace domain too :

L(i) = sL(P ) + L(P (t = 0))

L(i) =sP∞

(s+ ω0)+

ω0Ps(s+ ω0

+ (P∞ − P∞)

L(i) = P∞ + (Ps − P∞)ω0

s+ ω0

L(i) = ε0E[χ∞ + (χs − χ∞)ω0

(s+ ω0] (A.30)

For a constant applied voltage mathcalL(E) = 1sE and the admittance can

be expressed by :

L(Y ) =A

l

L(i)

L(E)=A

l[sε0χ0 + sε0(χs − χ∞

ω0

s+ ω0(A.31)

52

Spectroscopie d'impédance dematériaux à conduction ionique et électronique mixée (MIEC)

Attractives par leur faible latence et haute endurance comparé aux NAND Flash, les mémoiresde classe de stockage (SCM) doivent encore voir leur densité augmenter et leur prix de productionréduire avant d'être envisagées comme une alternative sérieuse. Une approche prometteuse est lasuperposition de réseaux de nouvelles mémoires non-volatiles. Cependant, cela requière des systèmesd'accès autres que ceux en silicium pouvant être fabriqués à basses températures (400°C), capablesde fournir un large courant ON pour écrire sur les cellules sélectionnées et de très faibles courantsde fuites pour toutes les autres cellules.

De nouveaux systèmes d'accès basés sur des matériaux à conduction ionique et électroniquemixées (MIEC) ontété étudiés et développés par IBM. Dans ces matériaux, le courant électroniqueest contrôlé par le déplacement des ions. Le comportement des matériaux à MIEC étudiés à IBMAlmaden est celui d'une diode dipolaire. Aussi bien les caractéristiques que l'endurance ( > 1010

cycles pour un faible courant) de ces systémes d'accès basés sur la MIEC ont déjà bien été étudiés.Malgré tout, la compréhension de la conduction électronique dans ces matériaux à MIEC reste in-complète. Pour ce projet, la spectroscopie d'impédance (IS) a été utilisée pour étudier ces nouvellesdiodes et le procédé électronique et ionique qui rend les caractéristiques de la MIEC si attirantes.

Dans un premier temps, des mesures d'impédance sont réalisées sur les échantillons utilisés pourles mesures IV faites sur C-AFM. Il s'est avéré que ces échantillons n'étaient pas propices aux mesuresIS. Aussi, de nouveaux échantillons avec une surface plus large ont été développés. En complément,l'AFM est délaissé au prot d'une plus simple sonde et dún nouveau protocole. Diérents problèmesont été identiés puis corrigés à l'aide d' échantillons purement résistifs ou capacitifs.

An d'expliquer la réponse en fréquence des échantillons testés, deux modéles théoriques de cir-cuit ont été développés. Les mesures d'impédance sont ainsi réalisées et les résultats comparés avecles dits modéles. Les premiers résultats concernant l'eet de la polarisation, de l'épaisseur des échan-tillons, du métal utilisé en guise d'électrode ou encore de l'annealing sont aussi prometteur qu'ils nesont intrigants.

Cependant de plus longues recherches doivent être menées sur l'IS avant que cette méthode nepuisse livrer de solides éléments à la compréhension des matériaux à MIEC. Les idées d'améliorationspouvant être apportées aux expériences seraient la création de petits et de larges échantillons surle même substrat ( an de vérier que les caractéristiques IV restent les mêmes), la réalisation decellules symétriques verticalement ( pour enquêter sur le comportement asymétrique lié à la polari-sation), le prolongement de l'IS à de plus hautes fréquences et biens d'autres perfectionnement dansle processus utilisé pour extraire les valeurs des composants du circuit théorique à partir des donnéesexpérimentales.

53

Spettroscopia d'impedenza dimateriali a conduzione mista ionica ed eletronica (MIEC)

Mentre le Storage Class Memory (SCM) sono interessanti poiché orono bassa latenza ed elevatadurabilitÃ rispetto alla tecnologia NAND Flash, le SCM non saranno fruibili dal mercato a menoche non vegano prodotte con la stessa elevata densità (e quindi baso costo) delle memorie Flash. Unapproccio promettente consiste nel mettere in pila piu' matrici di celle di memoria non volatili, dinuova generazione. Tuttavia, questo richiede un dispositivo di accesso, non in silicio, che possa esserefabbricato alle basse temperature (400 °C) normalmente usate per processi Back-End-Of-the-Line(BEOL), e allo stesso tempo garantire una elevata corrente ON per la scrittura delle celle selezionate,e una corrente di leakage molto bassa per le altre celle su un ampia gamma di valori di tensione.

Dei nuovi dispositivi di accesso basati su materiali a conduzione mista elettronica-ionica sonostati studiati e sviluppati dall'IBM. In tali materiali, la corrente elettrica é controllata dal moto degliioni. Nel materiale MIEC studiato dal gruppo di IBM Almaden, si osserva un comportamento similead un diodo bipolare. La caratteristica I-V, cosí come la durabilitá (1010 cicli bassa corrente) didispositivi di accesso MIEC sono ben noti. Non ostante questi risultati, la comprensione teoricadella conduzione elettronica in tali mteriali rimane incompleta. In questo lavoro é stata utilizzata laspettroscopia d'impedenza (IS) per studiare questi i processi elettronici e ionici che rendono attraentile caratteristiche dei materiali MIEC.

In primo luogo si é condotta la spettroscopia d'impedenza sugli stessi campioni usati per misureC-AFM della caratteristica I-V. Tali campioni, tuttavia, sono risultati inadatti per misure IS. E'stata quindi sviluppata una divesra struttura con un'area maggiore. Inoltre, si é abbandonato l'usodell'AFM in favore di una semplice sonda e una nuova procedura di misura IS. Diversi problemisono stati identicati e corretti tramite l'utilizzo di campioni di riferimento puramente resistivi ocapacitivi.

Al ne di spiegare la riposta in frequenza dei campioni misurati, sono stati sviluppati due mod-elli teorici circuitali. Sono state quindi eettuate le misure d'impedenza e i risultati comparati coni modelli teorici. Si riportano i primi risultati in funzione della tensione applicata,dello spessore deicampioni, del metallo usato come contatto, e dell'eetto dell'annealing.

Tuttavia, prima che la IS contribuisca alla comprensione teorica dei nuovi diodi MIEC saránecessario condurre ulteriori ricerche. Tra le idee per prosegiure questa ricerca ci sono: la creazionedi campioni di diversa dimensione sullo stesso substrato (al ne di vericare che le caratteristicheI-V restino simili), la realizzazione di celle a simmetria verticale (per investigare il comportamentoasimmetrco relativo al voltaggio applicato), l'estensione dell'IS a frequenze piú elevate e altri per-fezionamenti e automazioni sul processo di analisi dati impiegato per estrarre i parametri circuitaliteorici dai dati spettroscopici.

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Impedance Spectroscopy ofMixed Ionic Electronic Conduction (MIEC) materials

While Storage Class Memory (SCM) is attractive because it oers lower latencies and higherendurance than NAND Flash, SCM will not come to fruition unless it can be manufactured at thesame high density (and thus low cost) as Flash. One promising approach is to stack crosspoint arraysof novel Non-Volatile Memory in 3-D. However, this requires a non-silicon access device that canbe fabricated at the low (400°C) temperatures found in the Back-End-Of-the-Line (BEOL), yet stillprovide large ON currents for writing selected devices and ultra-low leakage currents for all otherdevices, over a wide range of voltages.

New access devices based on Mixed-Ionic-Electronic-Conduction (MIEC) materials have beenstudied and developed by IBM. In such materials, electronic current is gated by ionic motion. In theparticular MIEC material studied by the IBM Almaden group, the resulting behavior is a double, orbipolar, diode-like conduction. The IV-characteristics as well as the endurance (> 1010 cycles at lowcurrent) of such MIEC-based access devices have been well studied. Despite these accomplishments,the theoretical understanding of electronic conduction in this MIEC material remains incomplete.In this work, Impedance Spectroscopy (IS) is used to study the novel-diode MIEC material and thecoupled electronic and ionic processes responsible for its attractive electrical characteristics.

First, the same small samples used for C-AFM-based measurements of the IV-characteristicsare tested using Impedance Spectroscopy. Such samples are found to be unsuitable for IS, becausethe signicant tip capacitance overwhelms the much smaller signal due to the sample impedance.Hence, a new sample structure with much larger areas has been developed. In addition, a simple andexible non-AFM-based probe-station has been established, and a specic IS measurement proceduredevised. Various issues that can interfere with the spectroscopy are identied and corrected, usingpurely-resistive and purely-capacitive test samples.

Two theoretical circuit models for the MIEC device impedance are then developed to explainthe measured frequency response of the tested samples. Impedance spectroscopy is carried out onMIEC samples and the resulting curves are matched against these theoretical models. Intriguingand promising initial results are shown from studies into the eects of the voltage bias, the thicknessof the samples, the metal used as the top electrode, and the role of post-fabrication annealing.

However, signicant further research will need to be performed before IS can contribute to thetheoretical understanding to the novel-diode MIEC material. Ideas for future experimental improve-ments include the creation of both large and small samples on the same substrate (to ensure that thefamiliar IV characteristics are still being obtained), the implemention of vertically symmetric cells(to investigate the observed voltage-polarity-asymmetric behavior), the extension of IS to higher fre-quencies, and further renement and automation of the tting procedures used to extract theoreticalcircuit parameters from the IS data.

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Documents

Impedance Spectroscopy of novel Access Devices based on ...benji20.free.fr/IBM_MIEC/IBM_Report.pdfFirst, ince it is admited that current in MIEC materials is both due to electrons