H214 Journal of The Electrochemical Society, 0013-4651 ... · Two regimes were found in the growth...

Journal of The Electrochemical Society, 158 �3� H214-H220 �2011�H214

Growth Kinetics and Oxidation Mechanism of ALD TiN ThinFilms Monitored by In Situ Spectroscopic EllipsometryH. Van Bui,a,z A. W. Groenland,a A. A. I. Aarnink,a R. A. M. Wolters,a,b

J. Schmitz,a and A. Y. Kovalgina,*aMESA� Institute for Nanotechnology, University of Twente, Enschede Overijssel 7500AE, The NetherlandsbNXP Research Eindhoven, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands

Spectroscopic ellipsometry �SE� was employed to investigate the growth of atomic layer deposited �ALD� TiN thin films fromtitanium chloride �TiCl4� and ammonia �NH3� and the followed oxidation in dry oxygen. Two regimes were found in the growthincluding a transient stage prior to a linear regime. The complementary ex situ characterization techniques showed a goodagreement with the results obtained from SE measurements. A columnar structure of the as-deposited TiN film, which wascomposed of grains surrounded by amorphous material in between, was obtained. The X-ray photoelectron spectroscopy �XPS�analyses indicated low chlorine impurity content and slightly N-rich TiN films. The existence of an intermixed layer between thenitride and oxide during the oxidation was verified by the XPS depth profile analysis for a partially oxidized TiN film. Athree-layer optical model was constructed for SE in situ monitoring the oxidation. A four-regime oxidation was found for 15-nmTiN films whereas only two regimes were seen in the case of 5-nm films. A new oxidation mechanism was proposed to explainthe oxidation behavior of thin TiN films.© 2011 The Electrochemical Society. �DOI: 10.1149/1.3530090� All rights reserved.

Manuscript submitted October 26, 2010; revised manuscript received November 22, 2010. Published January 3, 2011.

0013-4651/2011/158�3�/H214/7/$28.00 © The Electrochemical Society

The continued down scaling of semiconductor devices leads tothe replacement of the common SiO2 gate with high-� materials. Inthe 90-nm complementary metal oxide semiconductor �CMOS� pro-cess, the SiO2 gate has reached a physical limit of five atomicmonolayers1 and the gate oxide leakage is ever increasing with de-creasing SiO2 thickness. For high-performance and low-powerCMOS applications, the combination of commonly used poly-Sigate and high-� dielectric is not well suited, bringing in the need forreplacement of poly gates with metal based ones.2 During the lastdecades, titanium nitride �TiN� has gained much interest because ofits low resistivity and compatibility with CMOS processes. TiN thinfilms are used as diffusion barriers3,4 and metallic gate electrodes.5-7

Mostly, sputtering and chemical vapor deposition �CVD� techniqueswere employed to deposit the TiN films. However, to achieve thinfilms with accurate thickness and composition control and excellentstep coverage, atomic layer deposition �ALD� has become an idealchoice for making such high quality films in the thickness range ofa few tens of nanometers. The ALD of TiN thin films can be realizedvia thermally activated processes, which are carried out at relativelyhigh temperatures, e.g., 350–450°C, using precursors such as tita-nium chloride �TiCl4� or TDMAT �Ti�N�CH3�2�4� and ammonia�NH3�.8-14 Recently, NH3 has been replaced by an H2–N2 plasmasource in plasma-assisted ALD, where TiN can be deposited at re-duced temperatures and higher growth rate.10,15

Although TiN is a hard metallic material with good thermal andchemical stability, it can be oxidized while exposed to air or oxidiz-ing ambient. W. Sinke et al.16 found that the TiN barrier perfor-mance between Si and Al to be improved when TiN was exposed toair prior to Al deposition. However, oxidation is not favorable inCMOS applications where the low electrical resistivity of TiN isrequired. Most of the studies on the oxidation of TiN were done onsputtered films with a thickness of several tens to hundreds ofnanometers,17-20 or more.21,22 In these studies, air or dry oxygen atatmospheric pressure was used for oxidation. Wet oxidation was alsodescribed in the literature.23 The ex situ characterization techniquesin the studies on dry oxidation led to different conclusions concern-ing the oxidation mechanism of TiN. Using X-ray photoelectronspectroscopy �XPS�, Saha and Tompkins18 found an intermediate�i.e., mixed� state between TiN and TiO2 during oxidation. The ex-istence of this intermediate state was illustrated in a later study ofHones et al.,20 in which spectroscopic ellipsometry �SE� and other

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complementary analysis techniques were employed. The results ofL. Soriano et al.,17 however, indicated an abrupt interface betweenTiN and TiO2 with complete phase separation upon oxidation.Therefore, questions regarding the oxidation mechanism of TiN arestill open. To further explore the applications of TiN in CMOS de-vices, the growth and stability of TiN films of several nanometers ofthickness need to be investigated in detail.

In this work, the growth kinetics and oxidation mechanism ofthin ALD TiN films were studied by in situ spectroscopic ellipsom-etry. Thin TiN films were grown by thermal ALD using TiCl4 andNH3 precursors. Subsequent oxidation of 5- or 15-nm-thick films indry oxygen was carried out in the same reactor without exposure toair. The oxidation was performed at a pressure of 10 mbar, i.e., inthe range commonly used for CMOS deposition processes, whichhas not been studied so far.

Experimental

ALD of TiN.— TiN thin films were deposited using TiCl4/NH3chemistry on standard p-type silicon �100� wafers with either nativeoxide or 108-nm-thick thermally grown oxide. An ALD cycle con-sisted of a 2-s pulse of TiCl4 precursor, followed by another 2-spulse of NH3. An N2 purge of 4 s was introduced in between thepulses, to remove the excess precursors and reaction by-products.The deposition was performed in our home-built single-wafer pro-cessing ALD reactor.24 Prior to the deposition, the wafer wascleaned by a standard cleaning process. The wafer was then loadedinto the ALD reactor via the load-lock. The load-lock allowed thereactor to be under high vacuum continuously. In the next step, thewafer was heated by a resistive heater embedded into the waferholder. The temperature was measured by a thermocouple, locatedclose to the wafer and controlled by a PID controller. On heating inN2 ambient, pressure in the reactor was kept at 10 mbar �i.e., highercompared to deposition� for 30 min. The TiN ALD processes werecarried out in the temperature range 350–425°C, at a total pressureof 10−2 mbar.

Oxidation of TiN.— Directly after deposition, temperature of thewafer was adapted �if necessary� and stabilized in N2 ambient at10 mbar, as mentioned above. TiN films, grown at 350 or 425°Cwith a thickness of 5 or 15 nm, were further oxidized, keeping the insitu SE monitoring. Dry oxygen with a constant flow of 25 sccmwas introduced to the reactor while the total pressure was kept at10 mbar. The oxidation was carried out at various temperatures, i.e.,325, 350, 400, and 425°C. There was no vacuum break betweendeposition and oxidation, eliminating the surface oxidation due tothe exposure of as-deposited TiN films to air.

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H215Journal of The Electrochemical Society, 158 �3� H214-H220 �2011� H215

Material characterization.— The deposition and oxidation ofTiN were in situ monitored by using a Woollam M2000 spectro-scopic ellipsometer, operating in the wavelength range between 254and 1688 nm, in combination with COMPLETEEASE modeling soft-ware. The ellipsometer was mounted on the reactor at an incidenceangle of 70° with respect to the substrate normal. SE in situ mea-surements as a function of time were taken at 2.5 s intervals. Thefilm thickness and surface roughness were measured by high reso-lution scanning electron microscopy �HRSEM� and atomic forcemicroscopy �AFM� techniques, respectively. The film compositionwas mainly analyzed by X-ray photoelectron spectroscopy. Othercomplementary characterization techniques including X-ray diffrac-tion �XRD� and X-ray reflectivity �XRR� were employed to investi-gate the crystalline structure and film composition. Transmissionelectron microscopy �TEM� was used to study the microstructure ofthe TiN film after deposition.

Results and Discussion

ALD of TiN.— Reaction mechanism of TiCl4/NH3 ALD chemis-try.— The TiCl4/NH3 ALD chemistry was previously described inseveral studies.12-14 The process is initialized by the reaction be-tween TiCl4 precursor and the hydroxyl �OH� groups which areformed due to the adsorption of water vapor on the substrate surface.This reaction leads to the formation of OTiCl3 terminations, whereCl atoms function as active centers for the next reaction when theNH3 reactant is admitted to the chamber. Both reactions release HClas a by-product, which is then purged out by the N2 flow. Based onits self-limiting reaction mechanism, ALD is claimed to have amonolayer-by-monolayer growth. However, it has been found thatthe growth of atomic layer deposited films is normally composed oftwo distinct regions including a transient regime prior to a lineargrowth. The transient regime implies that only a partial monolayercan grow at the early stage of the ALD process. In the initial reac-tion between TiCl4 and OH groups, there is a possibility that not allSi–OH sites will react, even in the subsequent cycles. The formationof a continuous or islanded morphology can be interpreted by thepreference of the reaction between the coming TiCl4 and either un-reacted Si–OH or previously created Ti–N–H sites.

Optical model for in situ spectroscopic ellipsometry.— In situ spec-troscopic ellipsometry is a nondestructive optical technique that hasbeen widely used for studying ALD processes.15,25,26 It uses polar-ized light and measures the polarized state of the beam reflectedfrom the sample as a function of wavelength in terms of ellipsom-etry parameters � and �. This technique makes it possible to havean observation of the film growth during deposition. In addition,optical and electrical properties of materials can be extracted fromthe measured ��,�� values.27,28 To interpret the measured data, anoptical model is required. The accuracy of the measurements ismathematically estimated by the mean squared error �MSE�, �,which represents the agreement between the measured data and thedata simulated by the optical model. Because the optical constantsof materials are usually unknown, modeling dielectric functions,constructing optical models, and fitting data are very important. Forthe ALD of our TiN, an optical model was simply built comprisingof a substrate with a TiN layer on top. The substrate is silicon �Si�with either native oxide or thermally grown SiO2. The thickness ofthe SiO2 layer was measured before deposition and the measuredvalue was used in the optical model during the growth and oxidationof TiN. The surface roughness is modeled by using the Bruggemaneffective medium approximation �EMA� consisting of 50% of voidsand 50% of material.27

The dielectric functions of TiN �shown in Fig. 1� were param-eterized by using the Drude-Lorentz model consisting of a Drudeterm and two Lorentz oscillators described as29-31

� = �� −�p

2

�2 − iD�+ �

j=1

2f j�0j

2

�0j2 − �2 + i j�

�1�


The Drude term describes the intraband absorption by free con-duction electrons in a material and is characterized by the dampingfactor D and the plasma frequency �p. The interband absorption isdescribed by Lorentz oscillators �the second term in Eq. 1� located atenergy position �0j with strength f j, and damping factor j whichrepresents the energy loss arising from various scattering mecha-nisms in a solid. The �� in Eq. 1 is a background constant, which isequal to or larger than unity to compensate for the contribution ofhigher energy transitions that are not taken into account in the Lor-entz term.31

The growth of TiN observed by in situ SE and material character-ization.— Figure 2 shows the growth of TiN on Si with native oxideat 425°C. It can be seen that the growth of TiN consists of the tworegimes mentioned above. The retarded nucleation at the beginningmight be due to not entire surface coverage by hydroxyl groups. Thelinear regime indicates an orderly growth which results mostly in atwo dimensional continuous film.32 The deposition rate then can beextracted from the slope of the linear-regime curve and is estimatedto be 0.26 Å/cycle in our case. No significant differences werefound between the growth of TiN on different types of Si substrates,e.g., p- or n-type Si with either native or 108-nm thermally grownoxide.

The agreement between the measured data ��exp,�exp� and thedata generated from the optical model ��mod,�mod� after deposition

Figure 1. �Color online� Real ��1� and imaginary ��2� parts of the dielectricfunctions of TiN according to Drude-Lorentz model.

Figure 2. �Color online� The growth of TiN on p-type �100� Si with nativeoxide substrate at 425°C for 1600 cycles. The inset shows the optical modelfor SE in situ monitoring.


H216 Journal of The Electrochemical Society, 158 �3� H214-H220 �2011�H216

is illustrated in Fig. 3. The calculated film thickness was 40 nmalong with a surface roughness of 1.57 nm after obtaining a good fitwith a minimized MSE value �2 � 1.3. These results were wellconfirmed by the followed ex situ HRSEM �Fig. 4� and AFM mea-surements. The film thickness and the surface roughness were deter-mined as 40 � 1 nm and 1.44 nm, respectively. This indicates that

(a)

(b)

Figure 3. �Color online� The fitting between measured and generated data of� �a� and � �b� after minimizing the MSE value.

Figure 4. HR-SEM image of a TiN layer grown at 425°C for 1600 cycles onSi with native oxide substrate.


the optical model and the parameterized dielectric functions used forin situ SE are in good agreement and, hence, can be trusted.

A TEM image of a 10-nm TiN film, grown at 425°C on siliconwith a 108-nm thermal oxide, is shown in Fig. 5a. A very thin layeron top with the thickness of 2 nm is referred to the native titaniumoxide �TiO2� which was formed due to the oxidation of TiN duringexposing to air. This image shows a columnar structure, which is ingood agreement with the model suggested by Saha and Tompkins.18

XRD measurements of as-deposited TiN indicated that the film has acubic crystalline structure oriented preferentially in the �200� latticeplane.

The composition of the 40-nm TiN film mentioned above �Fig. 2�was analyzed by XPS depth profiling. It is shown in Fig. 5b. A verysmall chlorine impurity content of less than 0.7% was found, indi-cating a highly pure TiN film. The presence of a large amount ofoxygen at the surface was due to the native oxide, which can be seenin the TEM image of Fig. 5a. The XPS analyses showed constantatomic concentration of both Ti and N throughout the film. Aslightly higher N-atomic concentration indicates the N-rich films.

Oxidation of TiN.— Optical model for in situ spectroscopic ellip-sometry.— To monitor the oxidation process of TiN in dry oxygenby in situ SE, an optical model must be constructed. A three-layermodel was previously used by Hones et al.20 for determining theoxide thickness by SE in the oxidation of 300- to 450-nm-thicksputtered TiN films. The model included an intermixed layer ac-counting for a partially oxidized film bulk underneath a pure oxidelayer, and a capping surface roughness layer. With the thickness of

(a)

(b)

Figure 5. �Color online� �a� TEM image of a 10-nm TiN film grown at425°C on Si with 108-nm thermally grown oxide and �b� XPS depth profileanalysis of a 40-nm TiN film deposited at 425°C on Si with native oxidesubstrate.



several hundred nanometers, TiN films were assumed to be opaque;therefore, the TiN layer was considered as the substrate in thatmodel. However, within several tens of nanometers of thickness, aTiN film is absorbing in the visible and near infrared wavelengthrange; as a consequence, it cannot be neglected in our optical model.

In this work, the construction of the optical model was ap-proached by analyzing the profile of a partially oxidized sample.Starting with a 15-nm TiN layer grown at 425°C on Si with a108-nm thermal oxide, the oxidation was performed for 3.5 h at325°C in dry O2 at 10 mbar. The time and the lower oxidationtemperature were intentionally chosen to enable only partial oxida-tion. Three different regions were found from the XPS spectra, asshown in Fig. 6a.

The first region indicates the presence of pure TiO2 with thebinding energy of 458.7 eV, which corresponds to the electronic

(a)

(b)

Figure 6. �Color online� �a� XPS spectra and �b� corresponding depth profileof a partially oxidized 15-nm TiN layer grown at 425°C on Si with a 108-nmSiO2. The TiN layer was exposed to oxidizing ambient for 3.5 h at 325°C.The inset in Fig. 4b shows the optical model for SE in situ monitoring.

Table I. Material density, film thickness, and surface roughness mea

Material

Density �g/cm3�

XRR X

Silicon substrate 2.33SiO2 1.99 1TiO 3.74
2

state of Ti2p in amorphous TiO2.18 The peak at 455.2 eV �i.e., thirdregion� represents the electronic state of Ti2p in TiN. The broadenedlines in between �i.e., second region� indicate the existence of amixture of nitride and oxide materials. The corresponding depthprofile of the above XPS analyses is shown in Fig. 6b. A gradualdecrease of the O-content and the corresponding N-content increasecan be observed, ending by a pure TiN layer with Ti:N � 1:1.Based on these results, the optical model for SE in situ monitoring isconstructed and schematically shown in the inset of Fig. 6b. Duringthe oxidation process, thicknesses of all the three layers were fittingparameters in the model.

The dielectric functions of amorphous TiO2 were parameterizedby employing the Tauc–Lorentz formulation,33 as shown in Fig. 7.The imaginary part �2 is modeled from the product of the bandgapof the materials and the Lorentz model, and can be mathematicallydescribed by following equations28

�2 =AEn0C�En − Eg�2

�En2 − En0

2 �2 + C2En2

1

Enif En � Eg �2�

�2 = 0 if En Eg, �3�

where A and C represent the amplitude and half-width of the �2peak, respectively. The bandgap Eg of amorphous TiO2 was thusdetermined from the dielectric function �2. The extracted value of3.25 eV was very close to the bandgap of 3.27 eV, as reported byZhang et al.34

The evaluation of this optical model was carried out by compar-ing the SE thickness measurements during oxidizing a 5-nm-thickTiN film with the later XRR analyses. The film density, thickness,and surface roughness obtained from XRR measurement were sum-marized in Table I. The TiO2 density was 3.74 g/cm3 and, thus,lower than 3.9 and 4.17 g/cm3 for anatase and rutile TiO2,respectively.35 Our value was close to the density of amorphousTiO2 �3.8 � 0.1 g/cm3�, as reported by Nakamura et al.36

Figure 7. �Color online� Dielectric functions of TiO2 according to Tauc–Lorentz model. As the �2-curve is described by Eqs. 2 and 3, this can be usedto extract the value of optical bandgap of amorphous TiO2. The latter corre-sponds to the first point where �2 � 0.

by SE and XRR.

Thickness �nm� Roughness �nm�

SE XRR SE

1088.9 1.1 1.3

sured

RR

07.59.0



Further results from the grazing incidence X-ray diffraction�GI-XRD� analyses, wherein no diffraction peaks of either anataseor rutile TiO2 were found, indicated that the obtained oxide wasamorphous. In summary, the good agreement between SE and XRRin the measurements of film thickness and surface roughness veri-fied the applicability of the optical model and the parameterizedoptical functions of TiO2. Both were employed for SE in situ diag-nostic in the follow-up oxidation experiments.

Oxidation of 15-nm-thick TiN films.— Figure 8a shows the oxida-tion at 425°C of a 15-nm TiN film grown at 350°C. Based on thethree-layer model, thickness of the oxide, intermixed, and nitridelayer were calculated using real-time calculation. The oxidationstarted immediately after introducing oxygen into the chamber. It iswell-known that oxidation of a material would normally include tworegimes, i.e., a linear regime at the beginning followed by a para-bolic regime controlled by the diffusion of oxidant.18-20 In thepresent case, however, the oxide-growth curve could be divided intofour stages. Starting with a very fast and short-in-time growth, theoxide-thickness increase slowed down in the second stage. Then, theoxidation continued with an enhanced rate and finally slowed downagain in the last stage. Similar behavior was found for the oxidationat 425°C of a 15-nm TiN film grown at 425°C �see Fig. 8b�.

The oxidation of the film grown at 350°C �Fig. 8a� was nearlycompleted within 6 min, which was confirmed by the XPS depth-profile analysis shown in Fig. 9. The oxide layer with a thickness ofapproximately 26.2 nm and a surface roughness of 2.1 nm was ob-

(a)

(b)

Figure 8. �Color online� The oxidation behavior at 425°C of 15-nm TiNfilms grown at either �a� 350°C or �b� 425°C. The films were deposited on Sisubstrate with a 108-nm thermal SiO2.


tained by SE ��2 � 2.4�. This thickness is in good agreement withtheoretical predictions, as a 1-nm-thick TiN layer would form ap-proximately 1.8 nm of TiO2 after being oxidized. This calculationwas done by assuming the densities and molecular weights of thematerials as listed in Table II.

Oxidation of 5-nm-thick TiN films.— For ultra-thin TiN films, wenoticed, however, a different oxidation behavior. For example, oxi-dation at 400 and 350°C of a 5-nm-thin TiN film grown at 425°C�see Fig. 10� provided the following observations. First, the oxide-and intermixed-layer thicknesses increased rapidly immediately af-ter starting the process. Second, the TiN layer thickness quicklydecreased, resulting in disappearance of the pure-TiN film long be-fore completing the oxidation. The oxidation of the TiN phase in theintermixed layer was further continued. In comparison with the15-nm-TiN oxidation, there were only two clear stages found for theultra-thin layers.

Influence of temperature on oxidation.— It is well-known that tem-perature has a strong influence on the formation and oxidation ofmaterials. To study this, a series of oxidation experiments was car-ried out, as listed in Table III. On the one hand, a change in oxida-tion temperature significantly affected the total oxidation time. Forexample, a 15-nm-thick TiN layer grown at 350°C could be com-pletely oxidized within 6 min at 425°C �sample 1�. When the oxi-dation temperature was reduced to 350°C, the oxidation could onlybe finished in 3 h �sample 2�. A similar trend was obtained for theultra-thin films �samples 5 and 6�. On the other hand, the rate ofoxidation depends on TiN-growth temperature, as illustrated bysamples 1 �Fig. 8a� and 3 �Fig. 8b�. For example, the total oxidationtime of the film grown at a lower temperature of 350°C was ap-proximately 5 min shorter than that of the film grown at 425°C. Thelatter could possibly be due to a better microstructure of the filmsformed at 425°C, e.g., bigger grains as well as fewer grain bound-aries and amorphous material. This led to the formation of denserfilms with a consequently lower oxidation rate.

Figure 9. �Color online� XPS depth profile of the film after oxidation at350°C of a 15-nm TiN film grown at 350°C. The profile confirms the nearlycomplete oxidation of the TiN. A minor N concentration at the interfacebetween SiO2 and TiO2 indicates a small amount of remaining TiN.

Table II. Molecular weight and density of TiN and TiO2.

MaterialMolecular weight

�g/mol� Density �g/cm3�

TiN 61.874 �Ref. 35� 5.21 �Ref. 35�TiO 79.866 �Ref. 35� 3.74 �this work�
2

SE data acquisition.



Proposed four-regime oxidation mechanism.— Based on the TiN-film microstructure obtained by TEM �see Fig. 5a�, four oxidationregimes �stages� can be proposed. These are shown in Fig. 11. FromFig. 5a, the initial TiN films comprised columnarlike grains sur-rounded by amorphous material in between. This is schematicallydepicted in Fig. 11a. When oxygen is introduced, the surface israpidly oxidized, resulting in a fast increase of TiO2 thickness �i.e.,stage one�. For example, this effect can be related to the electrontunneling phenomenon, which was suggested in previous studies onoxidation of silicon.37,38 The amorphous material between the grainsprovides favorable conditions �compared to the grain bulk� for oxy-gen to diffuse deeper into the film. This leads to the formation of theintermixed layer �Fig. 11b�. With increasing the oxidation time, thetop oxide and the intermixed layer will further expand from thesurface deeper into the bulk �i.e., stage two�. This process will resultin narrowing the TiN grains, i.e., sharpening the TiN pillars �Fig.11c�. At certain moment, a rapid lateral oxidation of the narrowedTiN grains, enhanced by their sharpness, will occur �i.e., stagethree�. The result is a fast disappearance of the large part of the TiNgrains �Fig. 11d�, accompanied by the rapid TiO2-thickness increase�see Fig. 8a and 8b wherein the kinetics is shown�. In our earlierexperiments on oxidation of mono-Si pillars, we observed the men-tioned enhancement of the oxidation rate while sharpening thepillars.39 The slow saturation regime �i.e., stage four�, limited by thediffusion of oxygen through the formed TiO2 layer, completes theoxidation process �Fig. 11e�.

The difference in oxidation kinetics between our current workand previous studies �see references above� can arise from the dif-ference in grain size, as well as from the relative fraction of amor-phous TiN between the grains. For a thickness of several hundrednanometers, the film is expected to have �a� bigger grains and �b�less amorphous TiN in between. This is due to the continuousgrowth of grains during deposition. Therefore, after the firstkinetics-limited stage, the diffusion of oxygen via both the grainboundaries and amorphous TiN does not play a significant role. Theoxidation occurs mainly in the downward direction and results in aparabolic regime.

In our study we also monitor a different oxidation behavior forthe thin and ultra-thin TiN films. For ultra-thin films �5 nm�, thetwo-regime behavior is observed compared to the four-regime be-havior of the thin films �15 nm�. Assuming that thinner films containfiner grains �i.e., more grain boundaries and, therefore, possibilitiesfor enhanced diffusion of oxygen� the diffusion of oxygen to theTiN/SiO2 interface can occur very quickly, causing a rapid disap-pearance of the pure-TiN layer. In this case, the third stage, observedfor the oxidation of a 15-nm-thick TiN layer, will start immediatelyafter the first stage. In other words, stage two �i.e., diffusion alonggrain boundaries and sharpening the TiN grains� can be omitted.Stage four, on the other hand, cannot yet occur as the TiO2 formedis still very thin.

Conclusions

In this work, we employed in situ spectroscopic ellipsometry tomonitor the growth and dry-oxidation processes at 350–425°C of�ultra� thin ALD TiN films. During the growth, two distinct regionsincluding a transient stage �i.e., incubation time due to nucleation�followed by a linear regime were found. As verified by HRSEM andAFM, Drude-Lorentz model could successfully be used to param-

e)

TiO2

e)

TiO2

Figure 11. �Color online� A schematicpicture describing the proposed four-regime oxidation of TiN.

Table III. Influence of deposition and oxidation temperatures ontotal oxidation time of differently thick TiN films.

SampleThickness

�nm�

Growthtemperature

�°C�

Oxidationtemperature

�°C�Oxidationtime �min�

1 15 350 425 62 15 350 350 1803 15 425 425 114 15 425 325 �2105 5 425 400 56 5 425 350 70

(a)

(b)

Figure 10. �Color online� Oxidation at �a� 400°C and �b� 350°C of 5-nmTiN films grown at 425°C. Oxygen was introduced in 5 min after starting the

a) b) c) d)

TiN Grain boundary Substrate

a) b) c) d)

TiN Grain boundary Substrate



eterize the dielectric functions of TiN. The as-deposited film had acolumnar structure comprised grains surrounded by amorphous TiNin between.

We further carried out the oxidation of as-grown 5- and15-nm-thick ALD TiN films in the same reactor and without vacuumbreak, accompanied by in situ SE monitoring. As verified by exter-nal XRR and XRD analyses, Tauc–Lorentz model provided accuratedielectric functions of TiO2. During the oxidation, the formed TiO2was amorphous with a density of 3.74 g/cm3 and an optical bandgapof 3.25 eV. The presence of an intermixed layer was indicated byXPS. A four-regime oxidation was observed for oxidation of15-nm-thick TiN films, whereas only two regimes were indicated forthe oxidation of the 5-nm-thick layers. Based on the TiN-film struc-ture analysis by TEM, we proposed four- and two-stage mechanismsto explain the observed oxidation behavior of 15- and 5-nm TiN,respectively.

Acknowledgment

The authors thank M. A. Smithers, M. D. Nguyen, E. G. Keim,and G. A. M. Kip �MESA�� for the HR-SEM, AFM, TEM, and XPScharacterizations, respectively. H. J. Wondergem �Philips Eind-hoven� is acknowledged for his XRR and GI-XRD analyses. Weexpress our gratitude to our colleagues I. Brunets, A. Boogaard, andJ. Lu for the help on SE and XRD measurements. This work isfinancially supported by the Dutch Technology Foundation STW,project 10017.

University of Twente assisted in meeting the publication costs of thisarticle.

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H214 Journal of The Electrochemical Society, 0013-4651 ... · Two regimes were found in the growth...

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Transcript of H214 Journal of The Electrochemical Society, 0013-4651 ... · Two regimes were found in the growth...