Properties of titanium nitride ﬁlms prepared by direct ...entsphere.com/pub/pdf/genia22apr2011/Properties of...Y.L. Jeyachandran et al. / Materials Science and Engineering A 445–446

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Materials Science and Engineering A 445–446 (2007) 223–236

Properties of titanium nitride films prepared by directcurrent magnetron sputtering

Y.L. Jeyachandran a, Sa.K. Narayandass a,∗, D. Mangalaraj a,Sami Areva b, J.A. Mielczarski c

a Department of Physics, Bharathiar University, Coimbatore 641 046, Tamil Nadu, Indiab Department of Physical Chemistry, Åbo Akademi University, FIN-20500 Turku, Finland

c Laboratoire ‘Environnement et Mineralurgie,’ UMR 7569 CNRS, INPL-ENSG, B.P. 40, 54501 Vandoeuvre-les-Nancy, France

Received 19 July 2006; received in revised form 28 August 2006; accepted 11 September 2006

bstract

Titanium nitride (TiN) thin films of different thickness were deposited by direct current (dc) magnetron sputtering under conditions of various2 concentrations (0.5–34%). The electrical, optical, structural, compositional and morphological properties of the films were studied and the

esults were discussed with respect to N2 concentration and thickness of the films. At low N2 concentration of 0.5% (of the total sputtering pressure.1 Pa), golden coloured stoichiometric TiN films were obtained and with increase in the N2 concentration non-stoichiometric TiNx phases resulted.owever, irrespective of the N2 concentration, the TiN stoichiometry in the films increased with increase in the film thickness. In the surface of

he films the presence of nitride (TiN), oxynitride (TiOxNy) and oxide (TiO2) phases were observed and the quantity of these phases varied withhe N2 concentration and thickness. The films of lower thickness were found to be amorphous and the crystallinity was observed in the films withncrease in the thickness. The crystalline films showed reflections corresponding to the (1 1 1), (2 0 0) and (2 2 0) orientation of the cubic TiN andlso features associated with TiNx phases. The transmission spectra of the films revealed the typical characteristics of the TiN films i.e. a narrow

ransmission band, however, the width varied with thickness, in the wavelength range of 300–600 nm and exhibited low transmission in the infraredegion. The TiN films deposited at low N2 concentration of 0.5% showed smooth and uniform morphology with densely packed crystallites. Withncrease in N2 concentration various characteristics such as needle type crystallization, bubble precipitates and after bubble burst morphologiesere observed in the films. However, at higher N2 concentration conditions, uniformity developed in the films with increase in thickness.2006 Elsevier B.V. All rights reserved.

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eywords: Titanium nitride films; dc magnetron sputtering; Properties; Nitroge

. Introduction

The nitride compounds of titanium (TiNx) are the uniqueaterials exhibiting both metallic (Ti–Ti) and covalent (Ti–N)

onding characteristics. The metallic properties are electricalonductivity and metallic reflectance; and the covalent bond-ng properties are high melting point, extreme hardness andrittleness, and excellent thermal and chemical stability. These

roperties of Ti and TiN have been frequently exploited forpplications in microelectronic devices [1], solar cells [2] and asrotective and decorative coatings [3]. Additionally, due to thentrinsic biocompatibility and hemocompatibility, TiN has beenuccessfully used as surface layers and electrical interconnects

∗ Corresponding author. Tel.: +91 422 2425458; fax: +91 422 2422387.E-mail addresses: [email protected] (Y.L. Jeyachandran),

[email protected] (Sa.K. Narayandass).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.09.021

centration; Thickness

n orthopaedic prostheses, cardiac valves and other biomedicalevices [4,5].

Various methods have been employed for TiN deposition6–10]. Among them sputtering techniques (dc and rf) are con-idered as most suitable methods and are being extensively usedor TiN deposition. The importance of sputtering methods is thathey involve a number of parameters such as nitrogen pressure,ase pressure, sputtering pressure, cathode power and substrate-arget separation in addition to substrate bias and temperaturehereby a number of combination of these parameters may besed to obtain high quality films with required properties.

However, a non-linear relationship was found to existetween the reactive gas pressure and other processing param-

ters exhibiting hysteresis effects thereby restraining the finalroperties of the films. Under low N2 concentrations goldenoloured stoichiometric TiN films were obtained [11–13], how-ver, controlling low N2 concentration was found to be difficult.

mailto:[email protected]:[email protected]/10.1016/j.msea.2006.09.021

2 e and Engineering A 445–446 (2007) 223–236

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Table 1Conditions employed for the preparation of TiN films

Parameters Values

(A) Deposition period from 10 to 15 minCathode power (CPw) 100 WSputtering pressure (SPr) 1.1 PaBase pressure (BPr) 4 × 10−4 PaSubstrate–target distance 100 mm

N2 concentration

0.5%3%6%11%17%22%27%34%

Thickness (nm)

0.5% 27% 34%

(B) Deposition period up to 30 min23 88 9542 142 13482 160

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24 Y.L. Jeyachandran et al. / Materials Scienc

t higher N2 concentrations, nitrogen oversaturation occurred inhe films sometimes leading to unusual effects such as nitrogenrecipitation at grain boundaries or as gas bubbles, superstruc-ure formation and partial crystallisation were observed [14–16].ence, due to the criticalness of nitrogen partial pressure, much

nterested has been paid to investigate its effect on TiN films17,18].

Furthermore, by sputtering methods, obtaining stiochiomet-ic TiN films were found possibly only under substrate biasingnd/or at higher temperature conditions [13]. However, the goaln microelectronics and industrial applications determines tworiginal constraints: (i) the substrate could not be heated to avoidtructural or compositional changes; (ii) the substrates must bender low or unbiased to avoid contact damages in electronicevices and also for coil-coating process. Therefore, there is areat interest to attain stoichiometric TiN films at low tempera-ures and without substrate biasing.

In the present work, by direct current (dc) planar magnetronputtering method nearly stoichiometric TiN films were obtainedn unbiased substrates at room temperature under conditionsf low N2 concentration. Together the effect of thickness and2 concentration on the properties of the films was studied.nalytical techniques such as resistance measurements, optical

ransmission spectroscopy, spectroscopic ellipsometry (SE), X-ay photoelectron spectroscopy (XPS), X-ray diffraction (XRD),canning electron microscopy (SEM) and optical microscopyere used to characterise the prepared films. The electrical, opti-

al, structural, compositional and morphological properties ofhe films were analysed with a particular emphasize on the effectf N2 concentration and film thickness.

. Experimental details

.1. Preparation of TiN films

TiN films were sputter deposited onto cleaned p-type (1 0 0)ilicon and glass substrates at room temperature from a titaniumetal target (75 mm diameter × 5 mm thick, 99.995% pure, PI-EM, England) mechanically clamped to the dc magnetron

athode of a conventional sputtering system (Vacuum Instru-ents Company, India). The silicon substrates were cleaned by

ollowing RCA (Radio Corporation of America) procedure [19]nd the glass substrates were cleaned by the method describedlsewhere [20]. Commercial argon (Ar2) and N2 gases (99%ure) were used as the sputtering and reactive gases respec-ively. The gas flow into the preparation chamber was controlledy two stage needle valves. Prior to Ti film deposition vacuumnd target conditioning were performed. The deposition cham-er was pumped down to the ultimate vacuum (4 × 10−4 Pa)nd repeatedly charged with Ar2 and pumped down in ordero minimize the residual gas components. The Ti target wasre-sputtered at a sputtering pressure (SPr) of 2 Pa and cathodeower (CPw) of 125 W to sputter out the surface oxide layer. The

re-sputtering was done until the Ti characteristic plasma glowdark blue colour) appeared. The TiN films were deposited underifferent N2 concentrations (0.5–34%) of the total SPr (1.1 Pa)nd of different thicknesses by fixing all the other parameters at

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ptimum values (details specified in Table 1). The optimum val-es of the fixed parameters were derived from our experience onitanium film deposition [20]. The effect of N2 concentration onhe properties of the films was studied by keeping the film thick-ess constant (∼60 nm) via adjusting the deposition period from0 to 15 min (section A in Table 1). Additionally, at low (0.5%)nd higher (27 and 34%) N2 concentrations, higher thicknesslms were prepared by extending the deposition period up to0 min (section B in Table 1).

The thicknesses of the films were measured using MBI tech-ique [21] and cross checked by SE measurements. The variationn the thickness values of the films as evaluated using the twoifferent techniques was found to be ±8 nm. In Table 1, the aver-ge thicknesses of films were presented. All the deposited filmsere found to be uniform over an area of 36 mm2 as observed

rom the sheet resistance measurements.

.2. Characterisation experiment details

All the characterisation experiments were made ex situ andt room temperature. For SE measurements the films depositednto silicon substrates were used and for all the other character-sations the films prepared on glass substrates were used. Sheetesistance and resistance versus temperature (R–T) measure-ents for the TiN films were made using a four point probe sys-

em (Scientific Equipment, Roorkee, India). A constant currentource and a digital microvoltmeter were used to apply current

nd measure the voltage respectively and a PID controlledven was used as the temperature source. The crystallographicharacteristics of the films were studied by XRD (PHILIPSW 3040 X-ray diffractometer) using Cu K�1 (λ = 0.1542 nm)

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Y.L. Jeyachandran et al. / Materials Scienc

adiation operated at 30 kV and 30 mA. The scan was per-ormed in continuous mode for a 2θ range of 10–80◦. Theptical transmittance spectra of the films were recorded from aV–vis–NIR spectrophotometer (VARIAN, Cary 500).For the TiN films of different thicknesses (23–153 nm) that

ere deposited at an N2 concentration of 0.5%, the chemi-al composition was obtained by XPS (KRATOS, XSAM800).he measurements were performed at a vacuum of 10−6 Pasing a Mg K� X-ray source (λ = 1253.6 eV). The UNIFITTUVersion 2.1) [22,23] software was used for peak fitting the high-esolution scans and for quantitative chemical analysis, applyingensitivity factors given by the manufacturer of the instrument.he high-resolution spectra were charge compensated by set-

ing the binding energy (BE) of the C (1s) contamination peako 284.4 eV. The morphology of the films was studied by SEMSiemens, UK). The images were taken at an accelerating voltagef 5 kV. The optical constants spectra of the films were obtainedrom the SOPRA GESP5 spectroscopic ellipsometer operatingn a spectral range 250–850 nm with variable angle of incidence.

For those films deposited at the other N2 concentrations, theollowing characterisation tools were used. The XPS systemmployed was Perkin-Elmer PHI 5400 ESCA System Spec-rometer using Mg X-ray source. The SEM instrument usedas Philips XL20 electron microscope. The SEM micrographs

ere taken at acceleration voltages of 10 and 15 kV. The opticalicroscopic images of the films were obtained from Leica (Q-in) optical microscope. The SE measurements of the films werebtained from a VASE spectroscopic ellipsometer (J. A. Wool-

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ig. 1. Sheet resistance, resistivity and temperature coefficient of resistance values aoncentration of 0.5%.

Engineering A 445–446 (2007) 223–236 225

am Inc.). The angle of incidence and the polarization azimuthere set at 70◦ and 135◦, respectively. The data were measured

n the wavelength range of 300–1100 nm.The use of different instruments (XPS, SEM and SE) was

ecause the characterisation of TiN films prepared at 0.5% N2oncentration and those prepared at all the other N2 concen-rations was performed at two different laboratories. However,he basic accuracy of the instruments was comparable and theesults obtained were within the instrumental error.

. Results

.1. Resistance studies

Fig. 1 shows the room temperature sheet resistance (R) valuesf the TiN film of different thicknesses deposited at the N2 con-entration of 0.5%. Also shown in the figure are the behaviour ofof the films with temperature (303–373 K), and the resistivity

nd TCR values of the films. The R value of the films decreasedrom 85 to 15 �/sq with the increase in thickness. Films of allhicknesses followed metal type R–T behaviour, that is, the Ralues increased with increase in temperature. The resistivityf the films was within ±20 of 210 �� cm and the TCR valuencreased from 0.002 to 0.05%/K with thickness.

The R values of the films deposited at higher N2 concen-rations (3–34%), in general, increased from 7.2 to 87.5 k�/sqith increase in N2 concentration and decreased with increase

n thickness. A low R value of 133 �/sq was obtained for the

nd resistance–temperature plots of films of different thickness prepared at N2

226 Y.L. Jeyachandran et al. / Materials Science and Engineering A 445–446 (2007) 223–236

Fig. 2. Resistance–temperature plot of the film of thickness 160 nm prepared atavr

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N2 concentration of 34%. Along the y-axis are the normalised sheet resistancealues. R is the sheet resistance value at any temperature (K) and R0 is the sheetesistance value at room temperature (303 K).

lm of thickness 160 nm deposited at N2 concentration of 34%nd a value of 260 �/sq was observed in the films of thickness42 and 134 nm prepared at N2 concentration of 27 and 34%espectively. All the films prepared at higher N2 concentration3–34%) showed semiconductor type R–T behaviour, that is, the

value decreased with increase in temperature. A typical R–Tlot of the film of thickness 160 nm deposited at N2 concentra-ion of 34% is shown in Fig. 2.

.2. Optical transmission studies

The optical transmission spectra of the films prepared at2 concentration of 0.5% are shown in Fig. 3. The films

xhibited a transmission band in the visible wavelength region

300–1000 nm) and a low transmission in the higher wavelengthegion. The transmission percentage and width of the transmis-ion band increased, and the transmittance peak shifted towards

ig. 3. Optical transmission spectra of the films prepared at N2 concentrationf 0.5%. The inset shows the shift in the peak transmittance wavelength withhickness.

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ig. 4. Optical transmission spectra of the films (60 nm) prepared at N2 con-entration of 3–34% and higher thickness films prepared at N2 concentrationsf 27 and 34%.

igher wavelength (376–475 nm) with decrease in the thicknessf the films.

The optical transmission spectra of the films deposited atigher N2 concentrations are shown in Fig. 4. The spectra of thelms of thickness 60 nm exhibited a transmission band in the

isible wavelength region (300–700 nm) and the transmissionncreased in the higher wavelength region. With an increase inhe N2 concentration, the transmittance of the films and broad-ess of the visible transmission band increased. At a high N2

Y.L. Jeyachandran et al. / Materials Science and Engineering A 445–446 (2007) 223–236 227

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The XPS survey spectra of normal unetched and 15 nm etchedsurfaces of the films of thickness 42–153 nm prepared at the N2concentration of 0.5% exhibited the characteristic Ti 2p, O 1s

Table 2Refractive index and extinction coefficient values of the TiN films at 632.8 nmas derived from and spectroscopic ellipsometry and optical transmissionmeasurements

N2 concentration (%) Thickness (nm) Optical constants 632.8 nm

n k

0.5

82 1.33 2.24112 1.34 2.07123 1.32 2.26153 1.42 2.16

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60

1.83 1.176 1.92 1.2611 1.82 1.0717 1.77 1.0722 1.81 1.1427 1.92 1.0234 1.83 0.92

27a88 1.62 1.08

ig. 5. Refractive index and extinction coefficient spectra of the films of variorepared at N2 concentrations of 3–34%.

oncentration of 34% the transmission band was completelyost and the films recorded high transmittance with an absorp-ion band edge at 350 nm. With increase in the thickness of thelms prepared at N2 concentrations of 27 and 34%, the transmit-

ance decreased and the narrowness of the visible transmissionand increased with relatively low transmission in the higheravelength region.

.3. Spectroscopic ellipsometry studies

The spectra of the optical constants such as refractive indexn) and extinction coefficient (k) of the TiN films as obtainedy SE are shown in Fig. 5. The films of different thicknessesrepared at the N2 concentration of 0.5% exhibited similar

and k spectral behaviour. A maximum in the n patternsf the films appeared at wavelength ∼300 nm and a mini-um at the wavelength range of 500–600 nm. The k plots

f the films showed a minimum in the wavelength range of00–400 nm with a steep increase with wavelength. Likewise,he films of thickness 60 nm prepared at various N2 concen-rations (3–34%) showed similar n and k spectral behaviourith a minimum in the n and k plots at wavelength ranges of00–530 nm and 372–450 nm, respectively. The n and k values

f the films at 632.8 nm as obtained from the Fig. 5 are presentedn Table 2. Also given in the table are the n and k values of thelms of higher thicknesses deposited at higher N2 concentra-

ions 27–34% as evaluated from the optical transmittance data24].

3

icknesses prepared at N2 concentration of 0.5% and films of thickness 60 nm

.4. X-ray photoelectron spectroscopy studies

142 1.46 1.2

4a95 1.78 0.89

134 1.69 0.89160 1.43 0.67

a Values derived from optical transmission data.


F face of the film of thickness 82 nm prepared under 0.5% N2 concentration condition.

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Table 3Assignments to the component peaks of the high resolution XPS scans of thefilms prepared at 0.5% N2 concentration condition

Peak Component Binding energy (eV) Assignment

Ti 2p3/2 I 458.1 TiO2II 456.1 TiOxNyIII 455.6 TiN

O 1s IV 529.8 TiO2V 531.6 O–H [Ti(OH)2 or H2O]

N 1s VI 396.1 TiNVII 398.1 O–N [TiO N ]

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ig. 6. XPS survey spectra of the (a) unetched and (b) 15 nm sputter etched sur

nd N 1s peaks at the corresponding binding energies 528.2,56.5 and 396.2 eV, respectively [25,26]. The surface of the filmsas etched in the XPS chamber by sputtering using argon gas.he typical survey spectra of the film of thickness 82 nm areresented in Fig. 6. The C 1s peaks in the spectra may be theontributions from organic carbon. The observed sodium (Nas) in the spectrum of the unetched surface may be from thelass substrate whereby Na atoms are highly mobile and so canasily diffuse to the surface of the films. Additionally, the Ar 2peak identified in the spectra of the etched surface may be fromhe adsorbed argon during etching or argon species incorporatednto the films during growth [27]. The elemental Ti/N ratios asvaluated from the spectra of the films are plotted in Fig. 7. Thei/N ratio decreased on surface etching. The Ti/N ratio was in

he range of 1.12–1.25 in the normal surface and 1.1–1.22 in theputter etched surface of the films.

From high-resolution XPS measurements of the normal sur-ace of the films, the spin orbit doublet Ti 2p1/2 and Ti 2p3/2eaks at binding energies 462.5 and 458.1 eV, respectively wasound in the Ti 2p spectra. The Ti 2p3/2 peaks included threeomponents whose peaks centered at 458.1 (I), 456.1 (II) and55.6 eV (III). Both the O 1s and N 1s peaks showed two com-

onents resolution centered at 529.8 eV (IV), 531. 6 eV (V) and96.1 eV (VI), 398.1 eV (VII), respectively. The typical high-esolution spectra of the film of thickness 82 nm are shown

Fig. 7. Ti/N ratio in the films prepared at N2 concentration of 0.5%.

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iO2: titanium dioxide; TiOxNy: titanium oxynitride; TiN: titanium nitride;i(OH)2: titanium hydroxide.

n Fig. 8. The most probable assignments to the origin of theomponents are presented in Table 3 [28–31] and the relativeercentage of the components as evaluated from the high res-lution spectra are presented in Table 4. The components IIInd VI of the Ti 2p3/2 and N 1s peaks respectively associatedith the TiN phases increased with the increase in thicknessf the films. The TiOxNy component (II) of the Ti 2p3/2 peakenerally, however a higher percentage in 82 nm film, remainedlmost constant and the OH− component (V) of the O 1s peakhowed a decreasing trend.

The survey spectra of the normal surface of the films of thick-ess 60 nm deposited under different N2 concentrations (3–34%)

nd films of higher thickness prepared at 22 and 34% conditionsxhibited the characteristic Ti 2p, O 1s and N 1s peaks sim-lar to that of the films of 0.5% N2 concentration condition.he high resolution Ti 2p peaks of the films displayed the spin

able 4elative percentage of the components of Ti 2p3/2, O 1s and N 1s peaks asvaluated from the high resolution XPS spectra of the films prepared at 0.5%

2 concentration condition

ilm thicknessnm)

Relative percentage of the components

Ti 2p3/2 peak O 1s peak N 1s peak

I II III IV V VI VII

42 67.8 20.6 11.6 51.2 48.8 52.6 47.482 57.1 28.7 14.2 56.5 43.5 65.6 34.423 63.9 21.7 14.4 58.2 41.8 69.4 30.653 61.0 22.3 16.7 67.5 32.5 81.8 18.2


Fig. 8. XPS high resolution spectra of the film of thickness 82 nm prepared at a N2 concentration of 0.5%.

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surement was carried out for the 523 K annealed film. The XPSsurvey and high resolution scan spectra of the 523 K annealedfilm are shown in Fig. 10. The characteristic Ti 2p, O 1s andN 1s peaks in the survey spectra and the doublet peaks in the

Table 5Assignments to the component peaks of the N 1s high resolution spectra of thefilms deposited under N2 concentrations of 3–34%

Peak Component Binding energy (eV) Assignment

VI 395.6 TiN

ig. 9. High resolution XPS scans of the films of thickness 60 nm prepared at4% condition.

rbit splitting characteristics and the Ti 2p3/2 peaks exhibitedhe components resolution corresponding to TiO2, TiOxNy andiN species. The O 1s peaks exhibited the typical TiO2 andH− components peak; and the N 1s peaks displayed three to

our (only for 3% condition) component resolutions at the bind-ng energies 395.6, 396.8, 399.1 and 400.2 eV. The typical highesolution scans of the films of thickness 60 nm prepared at 3nd 17% conditions, and 95 nm thick film of 34% condition arehown in Fig. 9. The most probable assignments to the compo-ents of the N 1s peaks are presented in the Table 5 [32–34].

The film prepared under 22% N2 concentration conditionshat showed bubble precipitate morphology (results presentedelow in Section 3.6) was subjected to annealing in argontmosphere (10 Pa) for 60 min at 523 and 723 K. XPS mea-

N

T

ncentrations of 3 and 17% and that of the film of thickness 95 nm prepared at

1sVII 396.8 O–N [TiOxNy]VIII 399.1 N–HIX 400.2 Adsorbed N2

iOxNy: titanium oxynitride; TiN: titanium nitride.


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Fig. 10. XPS survey spectra and high resolution scans of a film of thi

igh resolution Ti 2p peaks were observed. The Ti 2p3/2 peakxhibited showed only one resolved component correspondingo TiO2 phase and the O 1s peak showed two componentsssigned to TiO2 and OH− species. The N 1s peaks did nothowed a definite peak, however, two crests observed in theoisy background at the binding energies 399.4 and 402.2 eVay be assigned to the contributions from N–H and adsorbed

2 species, respectively [32–34].The relative percentage of the components as evaluated from

he high resolution spectra of the films of 3–34% conditionsre given in Table 6. Also given in the table are the relative

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able 6elative percentage of the components and elements on the surface of the films prepa

pectra

2 concentration (%) Relative % of the components

Ti 2p3/2 peak O 1s peak

I II III IV V

3a 88.4 11.6 – 77.8 22.26 72.0 17.9 10.1 67.2 32.81 69.2 18.5 12.3 52.9 47.17 70.1 21.5 8.4 60.1 39.92 72.2 18.3 9.5 56.2 43.87 76.5 17.8 5.7 52.8 47.22b – – – 62.2 37.84c 57.2 23.1 19.7 51.8 48.2

a For this film no TiN component in the Ti 2p3/2 peak was observed and in N 1s peb Data for the film annealed at 523 K.c Data for the film of thickness 95 nm while the other data are for the films of thick

s 60 nm prepared at N2 concentration of 22% and annealed at 523 K.

ercentage of the elements Ti, O and N present in the surfacef the films. In general, the TiN component (III) of the Ti 2p3/2eak decreased with increase in N2 concentration, however,he TiN component increased with increase in the thicknesss for the film prepared at a N2 concentration of 34%. TheiOxNy component (II) was almost constant with change in2 concentration. In the O 1s peak the percentage of TiO2

omponent (V) decreased and in the N 1s peak the O–Nomponent (VII) increased and the component (VIII) assignedo the adsorbed N2 species decreased with increase in N2oncentration. The elemental Ti and N elemental percentage

red at N2 concentrations of 3–34% as evaluated from the high resolution XPS

at.% of elements

N 1s peak Ti O N

VI VII VIII

25.8 24.2 35.0 22.2 70 7.846.6 28.3 25.1 24.8 57.9 17.344.0 32.0 24.0 22.8 58.5 18.741.1 36.1 22.8 21.5 61.9 16.646.0 36.5 17.5 21.6 63.3 15.143.2 34.8 22.0 22.3 61.6 16.1

– – – 20.3 76.6 3.143.5 34.5 22.0 22.2 63.8 14.0

ak four components were observed.

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ig. 11. X-ray diffractograms of the films prepared at a N2 concentration of 0.5

aried from 21.5 to 24.5 and 7.8 to 18.5, respectively withenerally a decreasing trend with increase in N2 concentration.

.5. X-ray diffraction studies

The XRD patterns of the TiN films prepared at 0.5% N2

oncentration and the films of higher thickness prepared at 27nd 34% conditions are shown in Fig. 11. For 0.5% condi-ion, an amorphous like XRD pattern with a broad peak inhe 2θ range 15–39◦ was obtained for the film of thickness

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able 7imensions of the morphological features of the TiN films of different thicknessicrographs

reparation conditiona Thickness (nm) Feature dimension

Particle size (nm)

.5%

23 1642 23

123 38153 34

1% –7% 602%2% (523 K) –2% (723 K) 60

7%88 32

142 85

4%95 25

160 60

a N2 concentration conditions.

d the films of higher thickness prepared at N2 concentrations of 27 and 34%.

3 nm. With increase in thickness of the films the intensityf the broad peak decreased and diffraction peaks at 2θ ∼ 22◦,27.3◦ and ∼36.7◦ resolved in the amorphous background pat-

ern. The peaks observed at 2θ ∼ 22◦ and ∼27.3◦ in the patternsf the films of thickness 42–112 nm may be due to the contribu-ions from the crystallites with substoichiometric phases (TiNx)

35,36]. The peak at 2θ ∼ 36.7◦ observed for the films of higherhickness (123 and 153 nm) could be associated with the (1 1 1)rientation of stoichiometric TiN crystal particles with cubictructure [37].

prepared at various N2 concentration conditions as obtained from the SEM

Needle crystals Bubble diameter

Length (nm) Width (nm)

– – –

120–800 25–62 –100–560 20–70 780–1000 nm

– – 300–2200 nm– – 69–550 �m

9–125 �m

– – –

– – –

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For higher N2 concentrations (3–34%), the films of thickness0 nm exhibited an amorphous diffraction pattern with a broadeak in the 2θ range of 15–39◦. In the Fig. 11, the broad peak alsoppeared in the films of higher thickness prepared at N2 concen-rations of 27 and 34%. Additionally, the higher thickness (88nd 142 nm) films prepared at 27% N2 concentration conditionxhibited diffraction peaks at 2θ ∼ 22◦, ∼24◦ and ∼43◦. How-ver, at N2 concentration of 34%, an amorphous pattern wasbserved for the films of thickness up to 134 nm and the 43◦iffraction peak was observed for the film of thickness 160 nm.evertheless, for the films of thickness 95 and 134 nm small

houlders at 2θ ∼ 43◦ and 62◦ were observed. The lower angleeaks may be associated with the particles of the substoichio-

etric phases [35,36]. The 43◦ diffraction peak was assigned

o the contribution from cubic TiN particles with (2 0 0) orienta-ion and the 62◦ resolution was assigned to the (2 2 0) orientation37].

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Fig. 12. SEM micrographs of the film prepared

Engineering A 445–446 (2007) 223–236

.6. Microscopic studies

From the SEM micrographs, the films prepared at 0.5% con-ition were found to possess uniform and void free morphology.he micrographs showed better particle resolution. The aver-ge particle size evaluated from the micrographs of the films ofhickness 42, 82, 123 and 153 nm as presented in the Table 7as found to be in the range of 18–42 nm.Films of thickness 60 nm prepared at higher N2 concentration

3–34%) showed varying characteristics such as smooth, needleype crystallization, bubble precipitation and bubble burst-like

orphologies. The low (3%) and high (34%) N2 concentra-ion conditions produced films with smooth morphology. At 6%

ondition cluster like features was observed in the smooth back-round morphous. The needle type crystallites were observedn the films for 11 and 17% conditions and the number of theeedle crystallites decreased with the increase in the N2 con-

at different N2 concentration conditions.

Y.L. Jeyachandran et al. / Materials Science and

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ig. 13. Optical microscope images of the annealed films of thickness 60 nmrepared at a N2 concentration of 22%.

entration. Large bubble precipitates in the films were observedor 17 and 22% conditions and the number of bubbles increasedith increase in the N2 concentration. The individual needle

rystallites and the bubbles were spaced by relatively smoothorphology. The length (L) and maximum width (Wmax) of the

eedle crystallites and the horizontal diameter (Dh) of the gasubbles observed in the films are presented in Table 7. At a con-ition of higher N2 concentration of 27% rough and non-uniformorphology resulted in the films. However, the higher thicknesslms prepared at N2 concentrations 27 and 34% were found toe relatively smooth and uniform. The average particle size asbtained from the high magnification (100× k) of the films ofhickness 88 and 142 nm prepared at 27% N2 concentration and5 and 160 nm prepared at 34% N2 concentration are presentedn Table 7. The typical SEM images of the film of thickness53 nm of 0.5% conditions, 60 nm of 11, 17, 22 and 27% con-itions and 95 nm of 34% condition are shown in Fig. 12.

The optical micrographs obtained for the 523 and 723 K filmsrepared at 22% N2 concentration are shown in Fig. 13. Veryarge bubbles were observed and the bubble diameter decreasedith increase in annealing temperature.

. Discussion

In this section, for the purpose of clarity, the results obtainedor the films prepared at 0.5% N2 concentration are discussed

thaT

Engineering A 445–446 (2007) 223–236 233

rst and then the results of the films deposited at higher N2oncentrations are dealt with.

From Fig. 1, the decrease in R value with thickness may behe thickness effect; however, a slight deviation in the chemicalature of the films, still retaining the metallic characteristics,as also evident from the other results. The increase in TCRalue with thickness reveals the existence of the chemical naturehanges in the films and improvement in metallic character withncrease in thickness of the films.

A narrow transmission window in the wavelength region00–600 nm and low transmission towards higher wavelengthegion are the typical characteristics of TiN films [38]. Fromhe transmission spectra of the films (Fig. 3) the transmissionand for the film of thickness 153 nm was observed in the wave-ength range of 300–500 nm and the band width wavelengthange increased together with a red shift in the peak transmissionTmax) wavelength with decrease in thickness. These featureslso show the existence of change in chemical composition ofhe films with thickness with a possibility of stoichiometric TiNomposition at higher thicknesses (>82 nm) [2,38]. Generally,he optical features of TiN arise from the free carriers in Tid band.herefore, an increase in the number of Ti vacancies results indecrease in the density of free carriers and shifts in the Tmax,

ssociated with the screened plasma frequency, to higher wave-ength thereby creating the red-shifted spectra [38,39]. Alsorom the spectra, the observed increase in transmittance inten-ity with decrease in thickness of the films may be the thicknessffect [38].

The optical quality of the films of 0.5% N2 concentration con-ition was found superior when compared to the earlier report2,38]. For example, the peak transmission percentage obtainedor a film of thickness 82 nm was 12.7% (Fig. 3), however, Duru-oy et al. [38] obtained a transmission percentage of 6.5% for thelms of thickness 90 nm prepared under a condition of substrateias 160 V and temperature 573 K. The superiority of the presentesults is in the fact that the films were prepared at conditionsithout substrate bias and temperature.In the optical constants spectra (Fig. 5) of the films, the

bserved maximum and minimum in the n and k plots, respec-ively at the wavelength range of 300–400 nm may be relatedo E2-type transitions associated with the transitions betweenybridized Np–Tid bands [40–42]. The gradual increase in thevalues towards higher wavelength and the minimum in the natterns may be associated with the Drude–Lorentz-type (inter-and) transitions [40]. The interband transitions may correspondo those between the Γ v15 valence and Γ

c12 conduction band

Γ v15 − Γ c12, E1 gap) [39]. The values of n and k in the rangef 1.32–1.42 and 2.07–2.26, respectively as obtained at a wave-ength of 632.8 nm (Table 2) compare well with values for TiNlms reported elsewhere [43].

From Fig. 7, the Ti/N ratio decreased with increase inhickness of the films. The decrease in Ti/N ratio shows themprovement in TiN stoichiometry of the films as evidenced in

he above studies. In coherence the TiN component in both theigh resolution Ti 2p3/2 peak and N 1s peak increased (Table 4)nd the TiO2 component (I) in the Ti 2p3/2 peak decreased.he observed spin orbit doublet in the Ti 2p peak (Ti 2p3/2 and

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i 2p1/2) suggests that oxidation was the predominant surfaceeaction in all the films [26]. Correspondingly a strong TiO2omponent at 529.8 eV was observed in O 1s peak of the filmsFig. 8). The II (456.1 eV) component in the Ti 2p3/2 peakas assigned to TiOxNy due to the observation of the O–N

omponent at 398.1 eV in the N 1s peak of these films [26].In the XRD plots of the films (Fig. 11), the observed broad

eak features in the 2θ range of 15–39◦ and the diffraction peakst the 2θ ∼ 22◦ and ∼27.3◦ may be attributed to the oxidationn the films and subsequent formation of substoichiometric TiNhases. The decrease in the intensity of the broad peak and thevolution of a peak at 2θ ∼ 36.7◦ corresponding to TiN phasehow the improvement in the crystallinity and also chemical stoi-hiometry in the films with increase in thickness as substantiatedrom the above discussion.

The morphological studies showed the presence of nanoscaleicrostructural features such as particle size (16–38 nm, seeable 7) in the films. The increase in particle size reveals theevelopment of better crystallinity in the films with increase inhickness. The particles would have been formed by clusters oftill smaller sized crystallites, however, the present magnifica-ion of the images revealed little about these features.

In the films that were prepared at higher N2 concentrations3–34%), the observed high R values (7.2–87.5 k�/sq) in thelms of thickness 60 nm may be due to a large deviation in theirhemical composition from TiN stoichiometry. The decreasef R value with increase in thickness of the films prepared at7 and 34% N2 concentration conditions may be attributed tohe thickness effect; however, significant improvement in sto-chiometry with thickness was evidenced from other studiesdiscussed below). The R–T plots of the films of all thicknessesere of semiconductor type, as observed in Fig. 2, in contrast toetal type conductivity of the TiN films. This shows that, even

f the higher thickness films are supposed to have improved TiNtoichiometry they still hold semiconducting nature.

From the optical transmission results (Fig. 4) the observedroad transmission band in the visible wavelength region andigher transmission towards longer wavelengths together withncrease of these features with N2 concentration reveal the exis-ence of non-stoichiometry (TiNx) in the films and also decreasen nitride stoichiometry with increase in N2 concentration. Thevolution of narrowness of transmission band and decrease inransmission towards higher wavelengths in the films of higherhicknesses prepared at 27 and 34% N2 concentration conditionshows the increase in nitride stoichiometry with thickness. Theecrease of transmittance intensity with increase in thicknessay be the thickness effect [2,38]. The n and k spectra of thelms of thickness 60 nm prepared at different N2 concentrationFig. 5) show the presence of E1 and E2 type transition character-stics. The increased n and decreased k values obtained for theselms at a wavelength of 632.8 nm (Table 2) when compared with

hat values obtained for the films prepared at 0.5% N2 concen-ration condition may be attributed to the compositional changes

43]. The composition of the film inherently affects the dielec-ric constant of the film thereby causing an associated change inhe optical constants [44]. The values presented for the films ofigher thickness prepared at 27 and 34% N2 concentrations are

eice

Engineering A 445–446 (2007) 223–236

ncertain because they were derived from the transmission datahat do not account the reflectance and absorption losses.

The XPS survey spectra and high resolution Ti 2p and O 1sFig. 9) peaks of most of the films exhibited similar characteris-ics as observed for the other films (TiN films, 0.5% condition)hat have been already discussed. However, in the N 1s peaks andditional component (VIII) at 399.1 eV was observed that wasssigned to the contribution from N–H species. The N–H bondsight have been formed through the activity of the surface N

toms with the adsorbed water molecules (H2O). In support, aonsiderable amount of O–H component (V) could be observedn the O 1s peaks of the films (Table 6). The H2O adsorption in thehese films may be high, than in films prepared at 0.5% N2 con-entration, due to the existence of chemical non-stoichiometryn the films. The non-stoichiometric compositions would haveavoured surface charges (Ti3+ defect states) thereby mediatingignificant H2O adsorption and causing the evolution of N–Homponent [45,46].

The film of thickness 60 nm prepared at 3% N2 concentrationhowed distinct XPS spectral characteristics (Fig. 9) comparedith the films prepared at other N2 concentrations. No signif-

cant TiN component in the Ti 2p3/2 peak was observed andhe N 1s peak showed four components. The low nitridationn the films may be due to high surface oxidation. The sourcef oxidation may be the nitrogen gas. As in the present caseommercial nitrogen gas was used and it may have a consider-ble percentage of oxygen impurities [47]. Since Ti have lowhreshold towards oxidation the available percentage of oxygenn the nitrogen gas together with the background residual quan-ity would have mediated the high surface oxidation of the films.orrespondingly the component IX at 400.2 eV evolved in the1s peak that was assigned, in literature, to adsorbed nitrogen

riginating from the release of nitrogen during oxidation of theiN component in the film [32].

At N2 concentrations higher than 3%, nitridation wasbserved in the films, however, the nitride percentage decreasedith increase in N2 concentration as partially evidenced fromptical transmission studies. This could be due to the effect ofxidation, as just mentioned, that may have sourced from thencreasing impurity component from the reactive gas. Corre-pondingly the increase in TiO2 component and decrease of TiNomponent of the Ti 2p3/2 with increase of N2 concentration>3%) could be observed from Table 6. At these N2 concentra-ion conditions (6–27%) the unreacted N2 present in the films

ay get buried in the films and sometime be precipitated asubbles. The bubble precipitates in the films prepared at 17 and2% N2 concentrations could be observed from the SEM imageshown in Fig. 12. As the case, the XPS spectra (Fig. 10) ofhe annealed film prepared at 22% N2 concentration conditionhowed no indication for the nitride component in both Ti 2p and

1s peaks. At the same time, the N 1s spectra revealed a com-onent at 402.2 eV that may be due to poorly screened nitrogentates [32]. At higher temperature the small nitride component

xisted in the film may have also been oxidized. However, withncrease in thickness of the films prepared at higher N2 con-entration (34%), as observed from Fig. 9 and Table 6 and alsovidenced from the optical transmission studies, the nitride com-

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Y.L. Jeyachandran et al. / Materials Scienc

onent increased. The increased nitridation at higher thicknessesay be due to the increased dissolution of nitrogen in the films.In the XRD studies, the amorphous nature observed in the

lms of thickness 60 nm prepared at different N2 concentrationonditions may be due to oxidation. Additionally, the glass sub-trates (amorphous character) and the room temperature prepara-ion condition may also be the reason for the amorphous naturen the films. The XRD patterns of the films of higher thick-esses prepared at 27 and 34% conditions (Fig. 11) showed theevelopment of nitride phases in the films with thickness, whichs in consistent with the optical and XPS results as discussedbove. However, a significant contribution from TiNx phasesas also observed, which may be responsible for the semi-

onducting nature in the films as obtained in the R–T studiesFig. 2).

From the SEM images of the films of thickness 60 nmFig. 12), the mechanism of formation and composition of needleype of crystallites in the films prepared at 11 and 17% conditionsre not clear at the present stage. Different microstructures suchs selective crystallization in an amorphous matrix and super-tructure formation in TiNx films were reported [15,16]. Theresent result of needle type crystallization is one of such dis-inct microstructure of TiNx films and effect of N2 concentration.he bubble structures observed in the films prepared at 17 and2% conditions may be due to the precipitation of unreacted N2nd also Ar incorporated during growth [14]. The non-uniformnd rough morphology observed in the films deposited at N2oncentration of 27% resemble something like bubble burst fea-ures. On annealing the film of 22% condition at 523 K, thencrease of bubble dimension (Fig. 13) may be due to the ther-al expansion of the gas precipitates and the decrease of bubble

imension on annealing at further higher temperature (723 K)ay be due to the burst of the large bubbles to smaller ones as a

esult of over expansion. Bubble precipitates in TiN films wereeported in the literature; however, the uniqueness of the presentesult is that the dimension of the observed bubbles was veryarge when compared to the reported values of 5–10 nm [14].

. Conclusions

TiN films of different thickness were prepared by dc mag-etron sputtering method under various N2 concentration condi-ions. The effect of thickness and N2 concentration (0.5–34%) onhe electrical, optical, compositional, structural and morpholog-cal properties of the films were studied by using resistance mea-urements, optical transmission spectroscopy, SE, XPS, XRD,EM and optical microscope techniques. The metal type electri-al properties, characteristic TiN optical transmission, structuraluality and chemical stoichiometry of the films improved withecrease in N2 concentration and increase in thickness. The filmsrepared at low (0.5–3%) and high (34%) N2 concentrationsndependent of the thickness and those films of higher thick-esses independent of the N2 concentration exhibited uniform

orphology with better microstructural properties. On the other

and, the films of thickness 60 nm that were prepared at N2 con-entrations of 6–27% exhibited various morphologies such asluster formation, needle type selective crystallization, bubble

[

[

Engineering A 445–446 (2007) 223–236 235

recipitation and non-uniform morphologies respectively withncrease in N2 concentration.

As a concluding remark, in the present work TiN films with aide range of chemical stoichiometry starting from highly oxi-ized composition through TiNx phase to stoichiometric TiNhase and with different morphologies were prepared and theesults were discussed. The possibility of obtaining stoichio-etric TiN films with good optical quality at room temperature

nd substrate unbiased condition has been demonstrated. Basedn this result together with the results of the effect of N2 concen-ration and thickness further investigations could be carried ono obtain good quality TiN films both optically and structurallyt conditions of low temperature and bias.

cknowledgement

We sincerely thank Dr. Tudor Jenkins, University of Wales,or his kind help in performing spectroscopic ellipsometry mea-urements for the samples of 0.5% condition and for the valuableiscussion on the results.

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Properties of titanium nitride films prepared by direct current magnetron sputteringIntroductionExperimental detailsPreparation of TiN filmsCharacterisation experiment details

ResultsResistance studiesOptical transmission studiesSpectroscopic ellipsometry studiesX-ray photoelectron spectroscopy studiesX-ray diffraction studiesMicroscopic studies

DiscussionConclusionsAcknowledgementReferences

Documents

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