Atomic Layer of Deposition of Ferroelectric HfO2

35TH ANNUAL MICROELECTRONIC ENGINEERING CONFERENCE, MAY 2017 1

Atomic Layer Deposition of Ferroelectric HfO2

Casey J GontaDepartment of Electrical and Microelectronic Engineering

Rochester Institute of Technology, 1 Lomb Memorial Drive, Rochester NY, USA

Abstract—Ferroelectric (FE) materials exhibit sponta-neous polarization making them particularly attractivefor non-volatile memory and logic applications. Recently,doped hafnium oxide has shown to be ferroelectric innature expanding its applications to these areas of in-terest. Ferroelectricity has been reported in atomic layerdeposition (ALD) of HfO2 with Al, Y, or Si dopants. Pre-vious work at RIT demonstrated functional ferroelectricfield effect transistors (FeFETs) using silicon doped HfO2

(Si:HfO2) as the gate dielectric.The new addition of a Savannah ALD system at

RIT has made deposition of doped HfO2 films possible.Recipes have been developed for deposition of aluminumdoped HfO2 using the hafnium precursor TDMAHf,tetrakis(dimethylamido)hafnium(IV), and the aluminumprecursor TMA, trimethyl aluminum. Transmission elec-tron microscopy (TEM) imaging and electron energy lossspectroscopy (EELS) were performed on the depositedAl:HfO2 films. TEM images verified that the target ofa 10nm film was achieved. The separate Al layers couldfaintly be seen in TEM images of unannealed samples.Capacitors with the deposited Al:HfO2 were fabricatedand tested. A small amount of ferroelectric behavior hasbeen observed however with low breakdown voltage aswell as high leakage current.

Index Terms—Atomic Layer Deposition, ALD, Hafnium,Ferroelectric, TEM, EELS

I. INTRODUCTION

MOORE’S law is the observed trend thatthe number of transistors per a given area

in an integrated circuit (IC) doubles about everytwo years. The most impactful innovations in themicroelectronics industry are those that keep thistrend alive. As transistors have become smaller andsmaller, silicon dioxide (SiO2) has been replaced asthe standard gate dielectric by materials that possessa higher relative permittivity or dielectric constant.These materials are referred to as high-k dielectrics.

Compared to SiO2 which has a dielectric constant(k) of 3.9, hafnium dioxide (HfO2) is a high permit-tivity dielectric material with a dielectric constant ofapproximately 25. HfO2 and other hafnium basedoxides have replaced SiO2 as the gate dielectric insilicon CMOS devices since the 45nm node.

Recently, it has been proven that by doping theHfO2 with a small atomic percent of materialssuch as aluminum (Al), yttrium (Y), or silicon,the dielectric shows ferroelectric behavior. Unlikeceramic materials like lead zirconate titanate (PZT)which have been known to show ferroelectric be-havior, HfO2 is CMOS compatible, scalable downto desired gate thicknesses, and atomic layer de-position (ALD) compatible. Ferroelectric HfO2 hasopened doors to many applications such as ferro-electric field effect transistors (FeFETs), negativecapacitance FETs (NC FETs), ferroelectric tunneljunctions (FTJs), and ferroelectric random accessmemory (FRAM).

II. THEORY

A. Ferroelectricity

The crystal structure of hafnium oxide, whetherbulk or thin film, is centrosymmetric and known tobe non-ferroelectric. However, with certain amountsof dopant in the hafnium oxide, the proper annealingconditions, and stress from a capping layer, the ma-terial falls into the non-centrosymmetric orthorhom-bic crystal phase leading to ferroelectric behavior.

Normal dielectric materials cannot retain apolarization without voltage. A polarization vsvoltage measurement of a capacitor with a normaldielectric would result in a line through the originwith a single slope. A ferroelectric capacitormeasured in the same fashion results in a hysteresis


loop instead of a line. It can be seen in Figure 1 thatin a measurement of this nature, two polarizationstates are present with zero applied voltage. Acharacteristic property of a ferroelectric materialis this spontaneous polarization and the amount ofcharge due to this polarization is referred to as theremnant polarization. The electric field required toeliminate this polarization, and further reverse it, iscalled the coercive field.

Fig. 1. Polarization vs voltage of a capacitor showing ideal ferro-electric hysteresis behavior. The remnant polarization and coercivefield are labeled.

B. Atomic Layer DepositionAtomic layer deposition (ALD) is a useful pro-

cess for the fabrication of microelectronics dueto its ability to produce accurate thicknesses anduniform surfaces in addition to high quality filmproduction using various different materials. In mi-croelectronics, ALD has been implemented as atechnique to deposit high permittivity gate oxides,high permittivity memory capacitor dielectrics, fer-roelectrics, and metals and nitrides for electrodesand interconnects.

ALD is a technique similar to that of chemicalvapor deposition, but has specific differences. ALDis a method in which a thin film is grown ona substrate by exposing its surface to alternatinggaseous species. Like chemical vapour deposition(CVD), it uses gaseous precursors, however theprecursors are never present simultaneously. Instead,the gaseous precursors are inserted as a series ofsequential, non-overlapping pulses. Each pulse istherefore a self limited reaction, terminating whensurface reactions with the precursor and all of theavailable bonds on the surface have taken place.

III. EXPERIMENTAL DETAILS

A. Material Development

The first reports on ferroelectric HfO2 were byNaMLab in Dresden, Germany in 2011. In 2015,previous studies were conducted at RIT by JoeMcGlone, a Microelectronic Engineering student,on ferroelectric field effect transistors (FeFETs)with depositions of the ferroelectric material done inpartner by NaMLab. In fabrication of these devices,the gate dielectric was silicon doped hafnium oxide(Si:HfO2). The devices were fabricated up to thepoint of the gate stack deposition. At this pointthe devices were sent to NaMLab for atomic layerdeposition of the Si:HfO2 and a capping layer ofTiN deposited by DC reactive sputtering. An annealof the layers was also conducted by NaMLab andthen returned. The final steps in fabrication of thedevices were then done at RIT. The polarization vsvoltage measurements of the RIT MFIS capacitorsas well as NaMLab MFM capacitors, done by JoeMcGlone, are shown in Figure 2.

Fig. 2. Comparison between the NaMLab metal-ferroelectric-metal(MFM) capacitor and the RIT processed metal-ferroelectric-insulator-silicon (MFIS) (N+ Si) capacitor.

The new addition of a Savannah ALD systemat RIT has made deposition of doped HfO2 filmspossible. Although the most successful dopant re-ported on is silicon, the ALD system at RIT iscurrently only equipped with hafnium, titanium, andzinc precursors. Recipes have been developed foraluminum doped hafnium oxide (Al:HfO2) ALD.


B. Process Details

For fabrication of capacitor devices with theAl:HfO2 material, the substrate was degeneratelydoped (ρ <0.0012 Ω·cm) p-type silicon for directcontact to the device when testing. For ALD of theAl:HfO2 material, the hafnium precursor TDMAHf,tetrakis(dimethylamido)hafnium(IV), and the alu-minum precursor TMA, trimethyl aluminum wereused.

Starting with a freshly cleaned silicon substrate,hydroxyl groups are present on the silicon surface.To initiate the ALD, the hafnium precursor is pulsedinto the chamber. Surface reactions then occur withthe hafnium precursor and the exposed hydroxylgroups. The result of this is the hafnium atomsbonded to the oxygen atoms from the hydroxylgroups and dimethylamine, (CH3)2NH, as a by-product. The chamber is then purged, removing allunreacted hafnium precursor and dimethylamine by-product. DI water is then pulsed into the chamber toreact with the rest of the exposed end groups fromthe initial precursor attached to the hafnium atoms.This reaction results in the formation of hafnium-oxygen-hafnium bridges. The chamber then con-tains unreacted water molecules as well as theby-product which is once again dimethylamine.Another purge is then performed to evacuate theunreacted water molecules and dimethylamine by-product, which then leaves the surface with exposedhydroxyl groups ready to accept the next layer ofhafnium atoms. This sequence is depicted in Figure3. This entire sequence is then repeated to formlayer after layer of HfO2. Each cycle deposits about0.94A of HfO2.

Once every ”N” cycles of hafnium precursor inthis sequence, the aluminum precursor is pulsedinstead of the hafnium precursor and reacts withthe surface in the same fashion. ”N” is calculatedbased on the desired dopant percent in the film. Forexample, a 4% Al doped film could be 96 pulses ofthe hafnium precursor and 4 pulses of the aluminumprecursor for a targeted 100A film. The by-productof the reaction between the aluminum precursortrimethyl aluminum, Al(CH3)3, and the exposedhydroxyl groups, as well as the subsequent reactionwith the water molecules, is simply methane: CH4.

For the capacitor device structures, a DC reactivesputtered titanium nitride (TiN) capping layer wasdeposited on top of the Al:HfO2. An aluminum layer

Fig. 3. Sequence depicting atomic layer deposition of HfO2. Eachtime this sequence is cycled, approximately 0.94A of HfO2 isdeposited.


for top contact was then deposited by DC sputteringand a simple single expose and develop lithographyprocess was performed. The final capacitor stackstructures were then formed by reactive ion etchingdown to the silicon and stripping the photoresist.

Films were deposited with aluminum dopant con-centrations ranging from 3% to 8% and differentannealing conditions after TiN deposition were per-formed ranging from 800C to 1000 for either 1 or20 seconds.

IV. RESULTS AND DISCUSSION

POLARIZATION vs voltage measurements wereconducted on the fabricated Al:HfO2 capacitors

as well as regular HfO2 capacitors with the samethickness dielectric. Figure 4 shows the polariza-tion vs voltage measurements of a 4% Al:HfO2

capacitor. It can be seen that when swept to ±1V,the device behaves like a normal capacitor (linear).Once voltage is swept higher than 1V, ferroelectricproperties (hysteresis) should be able to be ob-served. However, this measured device, like manyother devices tested, when a voltage higher than thisis applied, breakdown of the dielectric is seen andthe polarization curve approaches that of a resistor.Very inconsistently, devices tested resulted in shapesanywhere between that of what is depicted in Figure4.

Fig. 4. P-V measurements of a 4% Al:HfO2 capacitor. On the leftshows a measurement with an applied voltage of ±1V and on theright shows a measurement with an applied voltage of ±2V.

Transmission electron microscopy (TEM) imag-ing and electron energy loss spectroscopy (EELS)were performed on the deposited Al:HfO2 films as

well as the normal HfO2 films. Figure 5 shows aTEM image of a capacitor with normal HfO2 asthe dielectric. It can be seen in this image that thetargeted 10nm dielectric film was achieved. Figure6 shows an EELS line profile of the same deviceshown in the TEM image in Figure 5. This EELSline profile shows the elemental composition of thedifferent layers of the device, top aluminum, middleHfO2 of approximately 10nm, and bottom siliconsubstrate, proving that the desired materials weresuccessfully deposited.

Fig. 5. TEM image of a capacitor with a targeted 10nm HfO2 as thedielectric and a top aluminum contact.

Fig. 6. EELS line profile showing elemental analysis of the samedevice imaged in Figure 5. The left region shows aluminum (black),the middle region shows hafnium and oxygen (blue and greenrespectively), and the right region shows the silicon substrate (red).

Figure 7 shows a TEM image of an unanealedcapacitor device with 4% Al:HfO2. This image notonly shows that the deposited dielectric layer is alsoapproximately the targeted thickness of 10nm, butvery faintly, four distinct layers of what is assumedto be aluminum can be seen.


Fig. 7. TEM image of an unanealed 4% Al:HfO2 sample. Veryfaintly, four distinct lines can be seen which are assumed to be thefour deposited layers of aluminum in the film.

Fig. 8. EELS line profile of the same sample imaged in Figure7. Aluminum was omitted in this analysis. This elemental analysisshows a high concentration of oxygen and some silicon in-betweenthe HfO2 film and the silicon substrate. This is modeled as aninterfacial SiO2.

Figure 8 shows the EELS line profile of thesame device imaged in Figure 7, although aluminumconcentrations were omitted. This analysis showsan interfacial SiO2 in-between the Al:HfO2 layerand the substrate. By using equations 1 and 2, arelative permittivity of the Al:HfO2 can be extractedto compare to the known, reported value for HfO2.

Cox =ε · At

=ε0 · εr · A

t(1)

Ctotal =1

1CSiO2

+ 1CAl:HfO2

(2)

The measured total capacitance (Ctotal) of thestructure was approximately 190pF, the area (A) was

approximately 2x10−4cm2, the thickness (t) of theAl:HfO2 was approximately 1x10−6cm, the relativepermittivity of SiO2 (εr) is 3.9, and the knownpermittivity of free space (ε0) is 8.854x10−14F/cm2.Using these known values and the measured interfa-cial oxide thickness (tSiO2) of 2x10−7cm, Equation2 can be used where Equation 1 is used for thevalue of CAl:HfO2 . The relative permittivity of theAl:HfO2 calculated in this fashion is approximately23.9.

V. CONCLUSIONS AND FUTURE WORK

HAFNIUM oxide (HfO2) as well as aluminumdoped hafnium oxide (Al:HfO2) have both

been successfully deposited by atomic layer depo-sition at RIT for the first time. Capacitors with thedeposited dielectric materials were fabricated andtested. P-V testing of the devices however, resultedin low breakdown voltages observed at smallervoltages than ferroelectric behavior is reported tooccur. TEM images and EELS analysis have provedthat the deposited films are of the desired thicknessand material composition. The deposited dielectricsalso possess a calculated dielectric constant, relativepermittivity, of approximately 23.9 which is veryclose to that of the reported value of 25.

This project has opened the door to a greatamount of future work. Moving onward, fabricationvariations will be considered to limit interfacialoxide. An in depth study of the annealing conditionswill be done to achieve the proper orthorhombiccrystal phase. X-ray diffraction will be conductedto calculate the exact crystal phase the materialsexhibit after annealing.


ACKNOWLEDGEMENTS

I would love to thank Jackson Anderson, KarineFlorent, and Dr. Santosh Kurinec for all they havedone for me and this project thus far. I would belost without you!

I would like to thank Patricia Meller, BruceTolleson, and Dr. Karl Hirschman for all of theirwork on bringing ALD to RIT as well as for thetraining and running of the tool. I would also like tothank Dr. Ewbank and Dr. Pearson for their adviceand guidance all the way through senior design.

Also, I cannot forget a few others whose in-put, no matter how small, was valuable to thisproject. I would like to thank Spencer Pringle, ChrisO’Connell, Joe McGlone, David MacMahon (Mi-cron), NaMLab, and all of the rest of the amazingstaff of the SMFL at RIT!

REFERENCES

[1] E. Yurchuk, J. Muller, J. Paul, T. Schlosser, D. Martin, R.Hoffmann, et al., ”Impact of Scaling on the Performanceof HfO2-Based Ferroelectric Field Effect Transistors,” IEEETransactions on Electron Devices, vol. 61, pp. 3699-3706, Nov2014.

[2] T. S. Boescke, J. Muller, D. Brauhaus, U. Schroder, and U.Bottger, ”Ferroelectricity in hafnium oxide thin films,” AppliedPhysics Letters, vol. 99, p. 3, Sep 2011.

[3] T. Olsen, U. Schroder, S. Muller, A. Krause, D. Martin, A.Singh, et al., ”Co-sputtering yttrium into hafnium oxide thinfilms to produce ferroelectric properties,” Applied PhysicsLetters, vol. 101, p. 4, Aug 2012.

[4] U. Schroeder, S. Mueller, J. Mueller, E. Yurchuk, D. Martin, C.Adelmann, et al., ”Hafnium Oxide Based CMOS CompatibleFerroelectric Materials,” Ecs Journal of Solid State Science andTechnology, vol. 2, pp. N69-N72, 2013.

[5] Ekateriana Yurchuk, Electrical Characterisation of FerroelectricField Effect Transistors based on Ferroelectric HfO2 ThinFilms, Logos Verlag Berlin GmbH, Jun 30, 2015 - 238 pages

[6] J. Muller, S. Muller, E. Yurchuk, Ferroelectric Hafnium Oxide:A CMOS-compatible and highly scalable approach to futureferroelectric memories, Dresden, Germany

[7] K. Florent, Ferroelectric HfO2 for Ferroelectric Field EffectTransistor (FeFET) and Ferroelectric Tunnel Junction (FTJ)Applications, RIT

[8] J. McGlone, Ferroelectric HfO2 Thin Films for FeFET MemoryDevices, RIT

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Atomic Layer of Deposition of Ferroelectric HfO2