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Vacuum 86 (2011) 56e61

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Vacuum

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Experimental verification of model for prediction of coating compositiondeposited by sputtering using mosaic target in nitrogen

Kulwant Singh a,*, A.C. Bidaye b, A.K. Suri c

a FRMS, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, Indiab SES, MPD, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, IndiacMaterials Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

a r t i c l e i n f o

Article history:Received 21 May 2010Received in revised form6 April 2011Accepted 15 April 2011

Keywords:Magnetron-sputteringModelCompositionMosaicTieAlSticking coefficient

* Corresponding author. Tel.: þ91 22 25595378; faxE-mail address: singhkw@barc.gov.in (K. Singh).

0042-207X/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.vacuum.2011.04.013

a b s t r a c t

Sputtering, a physical vapor deposition technique, is widely used for preparation of compound coatings.Binary, ternary or multi component compound coatings deposited by sputtering find a variety ofapplications. The properties of these coatings depend strongly on the composition of the films.A mathematical model has been developed to predict the composition of the metallic constituents of thecoatings deposited by reactive sputtering using two metals mosaic target in the presence of a reactive gasatmosphere. The model has been worked out utilizing first order approximation and taking intoconsideration that there is no resputtering effect at the substrate. The model developed can also calculatethe percentage of covered areas of the target surfaces (target poisoning) with reactive gas flow. Themodel has been verified experimentally for TieAl mosaic target in a nitrogen gas environment utilizingexperimental data of sticking coefficient values.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Reactive sputtering is one of the techniques for deposition ofmetallic compounds [1e4]. Hard coatings of metal nitride havebeen successfully applied by sputtering respective metal in pres-ence of nitrogen for application on tools and many other areas [5]and [6]. Synthesis of the ternary compounds such as TieAleN,TieCreN, TieBeN, TieSieN, TieNbeN etc has been carried out byseveral investigators [7e11] and it has been observed that prop-erties of these composites depend strongly on the composition ofthe film and film structure. Therefore, controlling the filmcomposition and structure is desirable to synthesize the specific,tailor-made properties of the coatings. The important processparameters involved in a reactive sputtering include the gas pres-sure, bias voltage, ion current density and the voltage applied to thetarget [12] and [13]. The control of the reactive gas is especiallycritical for the optimization of film properties.

In the past, approaches have been made to predict the coatingcomposition deposited by different techniques. Möller [14] devel-oped a Monte Carlo code, which computes range profiles of

: þ91 22 25505151.

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implanted ions, composition profiles of the target and sputteringrates for a dynamically varying target composition. Carter et al. [15]proposed and analyzed a model for ion assisted deposition undernet growth conditions. Hubler et al. [16] presented a model toinclude the effects of sputtering, reflection of ions, multiple speciesbeam, charge exchange neutralization, and incorporation ofambient gas atoms and analyzed the data for the composition ofsilicon nitride, boron nitride and titanium nitride films as a functionof the arrival ratio of nitrogen (N2) ions to evaporant atoms. Harperet al. [17] studied the modification of thin film properties by ionbombardment during deposition. Most of these studies are forbinary compounds. There are very few studies for prediction ofcoating composition for ternary systems under dynamic conditionsof reactive gas [18] and [19]. Dreer et al. [18] presented a statisticalprocess model for the description and optimization of a techno-logical process and analyzed the data for AleOeN and SieOeNsystems. Eltoukhy et al. [19] described a model for reactive sput-tering of indium in N2eAr and N2eO2 plasma. Berg et al. [20]presented a basic reactive sputtering model for binarycompounds and correlated deposition rate and film compositionwith flow rates.

In this paper, a mathematical model has been proposed forternary system and experiments have been carried out to verify theprediction for TieAleN system. The model is for coatings deposited

B(0.5)A(0.5)Target Area

Coating

1- B1- A BA

To Pump

q q q q

Fig. 2. Sputter model e target surface areas.

K. Singh et al. / Vacuum 86 (2011) 56e61 57

by reactive magnetron sputtering utilizing mosaic targetcomprising of two metals. The model was used to predict thecomposition of the metallic constituents of the coatings for ternarycompounds (such as TieAleN, TieNbeN etc) with varying nitrogenflow. In the available literature the value of sticking coefficient ofreactive gases on metallic surfaces in the plasma environment hasmost often been assumed to be unity. Sticking coefficient is definedas the ratio of the number of adsorbate atoms that do adsorb, orstick, to a surface to the total number of atoms that impinge uponthe surface during the same period of time. In the present study, theactual experimental sticking coefficient value of reactive gas(nitrogen) on Al/Ti metals was used to test the model. The relativesticking coefficient values for nitrogen on e aluminium/titanium e

and niobium/titanium have been generated experimentally by theauthors recently [21] and [22].

2. Sputtering

Sputtering (Fig. 1) is a process in which high energetic ions andneutrals impinge the surface and transfer their momentum toatoms in the target; when this energy exceeds the binding energyof the atom, the atom leaves the target. For the model to be built-up, various aspects of the sputtering process have to be consid-ered; these are e target surface, substrate surface, various ions,neutrals, molecular species, cluster particles and parameters likecurrent density, incidence angle and target-substrate distance.

2.1. Assumptions

Various assumptions made are

� Nitride formation in the space between target and substrate isneglected, which is true for low pressures of nitrogen.

� Sputtering of higher order particles (clusters) is neglected.� Because of the low energy of Ar ions, sputtering occurs onlyfrom the top most atomic layer.

� Ion current density is uniformly distributed over the plasmazone of the target surfaces.

� Deposition of metal atoms with nitrogen takes place at thesubstrate.

� There is no resputtering effect at the substrate.

3. Theoretical model

The model has been worked out utilizing first order approxi-mation. The model developed can also calculate the percentage ofcovered areas of the target surfaces (target poisoning) withnitrogen flow. To build up the model, the scheme is presented:-

� Let there are 2 targets A & B of equal surface areas.� Sputtering gas is Ar & Reactive gas is N2.

� Species at the target surface are:

Cooling

Substrate holderVacuumpump

Magnetron

N S

D.C. Power su

N

a

Fig. 1. (a) Sputtering system

Toward the target: Arþ, Nþ2 , N2

From the target: MA, MB, MAN, MBN, N� Species at the substrate surface are: Arþ, Nþ

2 , N2, MA, MB, MAN,MBN, N.

� Neglecting higher order particles. Resulting major particles are,therefore,Arþ, Nþ

2 toward the targetMA, MB from the target to the substrate

� At substrate the presence of Arþ, Nþ2 play the role of biasing/

resputtering, which is ignored here.� IþAr >> IþN2

, The energy of N2 molecule further reduces to halfwhen it breaks down to N atom (N2 ¼ N þ N). Sputtering withNþ2 is, thus, ignored [23] and [24].

� Therefore, as a first order approximation, the followingprocesses can be considered to take place;-

� Sputtering due to Arþ at the target surface (N2 is present).� Deposition of metal atoms with nitrogen at the substrate.

3.1. Processes at the target surface

3.1.1. Nitrogen adsorption at the target surfaceAt any point of time (Fig. 2):

Let qAN ¼ Surface area of target A covered with nitrogen.

Let qBN ¼ Surface area of target B covered with nitrogen.

The surface areas of metals A and B not covered with N are,therefore:

Metal A ¼ 0:5� qAN

Metal B ¼ 0:5� qBN

At any point of time the flux adsorption can be given by:

On metal A; FAad ¼ FN22aA

�0:5� qAN

�(1a)

On metal A; FBad ¼ FN22aA

�0:5� qBN

�(1b)

Aluminium

Titanium

pply

Chamber

Substrate

Ground shieldTarget

Gases

Ground

Biasing

b

, (b) composite target.

K. Singh et al. / Vacuum 86 (2011) 56e6158

Where, a ¼ sticking coefficient, FN2is N2 flux and factor 2 is due to

diatomicity of N2 gas.

3.1.2. From kinetic theory of gases the N2 flux [20] and [25] is

FN2¼ PN2

=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið2pkTMÞ

q¼ K1PN2

(2)

Where, k ¼ Boltzmann constant, T ¼ Temperature, M ¼ wt of N2molecule

PN2¼ Partial pressure of N2 and factor K1 ¼ 1=

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2pkTM

p

3.1.3. Nitrogen desorption from the target surface

From A; FAdes ¼ SAMN JTqAN (3a)

From B; FBdes ¼ SBMN JTqBN (3b)

Where, JT ¼ Target current density and SMN ¼ Sputtering yield ofcompound

3.1.4. Metals sputtering flux from targets

From A; FAM ¼ SAM JT�0:5� qAN

�(4a)

From B; FBM ¼ SBM JT�0:5� qBN

�(4b)

3.2. Nitride coverage on targets

At steady state 1(a) ¼ 3(a) and 1(b) ¼ 3(b).From 1(a) ¼ 3(a), we get

FN22aA

�0:5� qAN

�¼ SAMN JTq

AN

qAN ¼ 1=h2þ

nSAMN JT=

�aAFN2

�oi(5a)

Similarly from 1(b) ¼ 3(b), we get

qBN ¼ 1=h2þ

nSBMN JT=

�aBFN2

�oi(5b)

Replacing FN2form Eq. (2) in Eq. (5)

qAN ¼ 1=h2þ

nSAMN JT=

�aAK1PN2

�oi(6a)

qBN ¼ 1=h2þ

nSBMN JT=

�aBK1PN2

�oi(6b)

The Eq. (6) gives the coverage of nitride formation on the targetsurfaces with respect to PN2

. Therefore, nitride coverage (%) on thetarget surface can be graphed with respect to PN2

and theoreticallypredicted.

3.3. Fluxes from only metallic portions of the targets

Putting the values from Eq. (6) in Eq. (4)

FAM ¼ SAM JTf0:5� 1=h2þ

�SAMN JT

�.�aAK1PN2

�io(7a)

FBM ¼ SBM JTf0:5� 1=h2þ

�SBMN JT

�.�aBK1PN2

�io(7b)

Eq. (7) represents the metallic fluxes leaving from the targetsA & B.

3.4. Fluxes from metallic and nitrided both portions of the targets

Eq. (7) accounts for fluxes from only metallic portions of thetargets. When compound fluxes sputtered from the targets are alsotaken into consideration, total flux becomes:-

FAT ¼ FAM þ FAMN (8a)

FBT ¼ FBM þ FBMN (8b)

FAT ¼ SAM JT�0:5� qAN

�þ SAMN JTq

AN (9a)

FBT ¼ SBM JT�0:5� qBN

�þ SBMN JTq

BN (9b)

Replacing qAN and qBN from Eq. (6), we get total flux

FAT ¼ JTnSAM

�0:5� 1=

h2þ

�SAMN JT

�.�aAK1PN2

�i�

þSAMN=h2þ

�SAMN JT

�.�aAK1PN2

�io(10a)

FBT ¼ JTnSBM

�0:5� 1=

h2þ

�SBMN JT

�.�aBK1PN2

�i�

þ SBMN=h2þ

�SBMN JT

�.�aBK1PN2

�io ð10bÞ

3.5. Total metallic constituents of coating

Fraction of constituents

A ¼ FAT =�FAT þ FBT

�(11a)

B ¼ FBT =�FAT þ FBT

�(11b)

Eqs. (10) & (11) give the composition prediction of metallicconstituents of sputtered coatings with respect to partial pressureof nitrogen. The graphs can be plotted for qAN, q

BN and composition Vs

PN2and the values can be theoretically predicted with PN2

.

4. Testing the proposition/model

4.1. Experimental

The experimental system consisted of a magnetron sputteringsystem (Fig. 1). Coatings were deposited on to the stainless steelsubstrates. For testing the model (TieAl)N coatings were depositedat various parameters and coatings were analyzed for compositionby energy dispersive X-ray (EDX) analysis techniques. TieAleNfilms were deposited by reactive D.C. magnetron sputtering ina custom built unit shown in Fig. 1a. Fig. 1b shows the Ti/Al mosaictargets used. Coating system consisted a vacuum chamber sup-ported by a diffusion pump backed by a rotary pump which couldgive a base pressure of better than 6 � 10�4 Pa. Magnetron wasattached beneath a water-cooled copper cathode on whicha 160 mm target was screwed and the chamber was grounded. Anadjustable height substrate holder was situated about 60 mm away

Plasma Zone

R2

R1

Fig. 3. Plasma zone on the target surface.

Table 1Partial pressure of nitrogen.

N2/Ar flow % PN2(10�2 Pa)

2.5 1.03605 1.99410 3.98415 5.85620 7.74925 9.51230 11.07935 12.55140 13.998

K. Singh et al. / Vacuum 86 (2011) 56e61 59

in front of the target onwhich plane substrates were mounted. Thesubstrates were ultrasonically pre-cleaned sequentially using anaqueous alkaline bath, ethanol and acetone before being placed inthe vacuum chamber. Gases after passing through moisture andoxygen traps were introduced into the chamber through flowmeters. Substrate holder was insulated from the chamber and waskept either floating or connected to a biasing power supply. Totalpressure of the deposition chamber was kept at approximately5 � 10�3 mbar (z0.5 Pa). Target current was kept at 0.6 Amp.Nitrogen to argon flow ratio was varied from 0 to 40%. The power tothe target was supplied by a stabilized D.C. power supply(0e1000 V, 6 Amp maximum). No external heating was applied tothe substrate. Sputter yields of Ti, Al and their nitrides were foundout experimentally. The sputter yields S (atoms/ion) were deter-mined experimentally using empirical formula [26] for 500 eV Arþ

ions.

S ¼ 105ðW=AItÞWhere, W ¼ weight loss (gm), A ¼ atomic weight, I ¼ ion current(amp), t ¼ sputtering time (sec).

4.2. Calculations

K1 : K1 ¼ 1=Oð2pkTMÞ

K1 ¼ 1=O�2� 3:14� 1:38� 10�23 � 300� 28� 1:66� 10�27

�

¼ 2:876� 1022sec=ðkg:mÞ

Table 2Flux from metallic portions and total flux (aTi ¼ 1, aAl ¼ 1).

Flux from metallic portion

PN2 10�2 Pa FMTi 1018/m2.sec FM

Al 1018/m2

1.036 14.61 44.261.994 7.81 24.163.984 4.18 13.095.856 2.94 9.237.749 2.31 7.269.512 1.93 6.0711.079 1.67 5.2712.551 1.49 4.7013.998 1.35 4.27

JT : Target current density JT ¼ I=ðAe� qÞWhere, I ¼ Target current (Amp), q ¼ electronic charge

Ae ¼ Effective sputter area (Fig. 3) ¼ ðp� R21 � R22Þ

JT ¼ 0:6Amp=�94:25� 10�4m2 � 1:6� 10�19Amp:sec

�

¼ 3:98� 1020=m2:sec

PN2: Partial pressure of N2 in the chamber was calculated by

taking the mass ratio of nitrogen gas to total gases present in thechamber. Amount of nitrogen gas present in the chamber wascalculated by deducting the amount of nitrogen reacted with thesputtered metals from the nitrogen amount passed in to thechamber. Nitrogen amount reacted with the sputtered metals wasobtained from the nitrogen analysis of the coatings deposited atvarious nitrogen flow rates. The partial pressure of nitrogen in thechamber (after deducting the amount of nitrogen reacted with thesputtered metals) at various N2 flows is shown in Table 1.

Nitride coverage ðqNÞon target : qTiN & qAlN

qTiN ¼ 1=�2þ ðSTiN JTÞ=

�aTik1PN2

��and

qAlN ¼ 1=�2þ ðSAlN JTÞ=

�aAlk1PN2

��

Sputter yields of Ti, Al and their nitrides were found outexperimentally. The values were:STi ¼ 0.51, SAl ¼ 1.05, STiN ¼ 0.10 &SAlN ¼ 0.154

The values matchedwith the available literature values [27e30].Taking the assumed unity value for sticking coefficients [18] and

[31] as

aNTi ¼ 1 & aNAl ¼ 1

The sputtered flux frommetallic portions of the targets and totalsputtered flux (from metallic as well as nitrided portions of the

Total sputtered flux

.sec FTTi 1018/m2.sec FT

Al 1018/m2.sec

44.90 88.8339.43 71.6736.52 62.2335.52 58.9335.01 57.2634.71 56.2434.50 55.5634.35 55.0734.24 54.71

0 2 4 6 8 10 12 14 160.43

0.44

0.45

0.46

0.47

0.48

0.49

0.50

-2PN2 (Pa x 10 )

Ti Al

Ti= Al=1

Nθ

α α

Fig. 4. Nitride coverage for Ti and Al targets (aNTi ¼ 1, aNAl ¼ 1).

Table 3Total Al flux (aNTi ¼ 1; aNAl ¼ 0.4).

PN2� 10�2 Pa FT

Al 1018/m2.Sec

1.036 130.361.994 97.743.984 77.495.856 69.997.749 66.089.512 63.6811.079 62.0612.551 60.8913.998 60.01

K. Singh et al. / Vacuum 86 (2011) 56e6160

targets) at different partial pressure of nitrogen (chamber pressure)has been shown in the Table 2.

Fig. 4 shows the nitride coverage of Ti and Al targets withincreasing partial pressure of nitrogen. Both metals show a similartrend of increasing poisoned area of target surface with theincrease in nitrogen partial pressure. Fig. 5 graphs the compositionof Ti and Al in the coating vis-à-vis partial pressure of nitrogenconsidering sputtering taking place from metallic portions of thetargets as well as composition prediction due to the sputteringtaking place frommetallic and nitrided both portions of the targets.

5. Discussion

In the calculations above, the nitrogen sticking coefficients forTi and Al both have been assumed to be unity (as referred in theprevious literature). According to thermodynamic data, the valuesof DG0

f for TiN and AlN are �308.3 and �287.0 kJ/mol, respectively.Nitrogen must therefore first react with Ti before Al. This differencein reactivity could probably affect the nucleation during thedeposition of TiAlN, their composition and subsequently influence

0 2 4 6 8 10 12 14 16

25

30

35

40

45

50

55

60

65

70

75

Metallic portion flux Ti Al

Com

p. (a

t%)

PN2 (Pa x 10-2 )

Total sputtered flux Ti Al

Fig. 5. Composition prediction considering flux from metallic portions of the targetsand total flux (aNTi ¼ 1, aNAl ¼ 1).

the microstructure and property. Theoretical values for equal areasof Ti and Al targets without nitrogen gas are predicted to give 35 at%Ti and 65 at% Al. Therefore, it is unlikely that the nitride coverage forTi and Al will follow the pattern as depicted in Fig. 4. Similarly, it isunlikely to follow the prediction of coating composition as stipu-lated in Fig. 5 based on the sticking coefficient values assumed to beunity for both of the metals simultaneously. As compared to Al, Tiexhibits higher reactivity and therefore higher sticking coefficient(aTi) to nitrogen is expected. Therefore, the sticking coefficient ofnitrogen on Ti and Al was found out experimentally by the authorsrecently replicating the actual sputter conditions [21]. The relativesticking coefficient of nitrogen on Al to Ti (aAl/aTi) was found to varybetween 0.37 and 0.42 under various treatment conditions.Therefore, taking the average approximate value of sticking coef-ficient for nitrogen on Al/Ti (aAl/aTi) as 0.4, all the data above havebeen recalculated.

Nitride coverage and total sputtered flux for Ti target willremain same; however, the values will change for Al target. Data fortotal sputtered flux for Al emanating from metallic and nitridedportions of the targets has been shown in Table 3. Fig. 6 shows thenitride coverage of Al target with increasing partial pressure ofnitrogen for sticking coefficient of aNTi ¼ 1 and aNAl ¼ 0.4. Predictedcoating composition Vs partial pressure of nitrogen has beengraphed in Fig. 7 for sticking coefficient of aNTi ¼ 1 and aNAl ¼ 0.4along with the experimental values obtained for Ti and Al analysisof the coatings, deposited at various nitrogen flow. The compositionvalues nearly correspond to the theoretical values calculated usingsticking coefficient as aNTi ¼ 1 and aNAl ¼ 0.4. Therefore, it is unre-alistic to assume the sticking coefficient as unity for both themetalssimultaneously. The proposed theoretical model is, thus, able topredict the composition of the metallic constituents of the coatings

0 2 4 6 8 10 12 14 16

0.36

0.38

0.40

0.42

0.44

0.46

0.48

0.50

N

PN2 (Pa x 10-2)

Al=0.4a

q

Fig. 6. Nitride coverage for Al target at aAl/aTi ¼ 0.4.

0 2 4 6 8 10 12 14 1620

30

40

50

60

70

80

Com

p. (a

t%)

PN2 (pa x 10-2)

Ti (Theoretical) Al% (Theoretical) Ti% (Experimental) Al% (Experimental)

Fig. 7. Theoretical (aNTi ¼ 1, aNAl ¼ 0.4) and experimental compositional values Vs PN2.

K. Singh et al. / Vacuum 86 (2011) 56e61 61

obtained at various partial pressures of a reactive gas (herenitrogen) utilizing composite target during sputtering. However,more realistic values of sticking coefficient must be taken in toaccount rather than assuming them to be unity for the metalsystems being used in the composite target.

6. Conclusion

A theoretical model has been proposed to predict the composi-tion of the coatings deposited by sputtering using two metalsmosaic target in the presence of a reactive gas (nitrogen) atmo-sphere. The model has been worked out utilizing first orderapproximation and taking into consideration that there is noresputtering effect at the substrate. The model developed can alsocalculate the percentage of covered areas of the target surfaces(target poisoning) with partial pressure of reactive gas. The modelhas been tested experimentally for TieAl system under nitrogen gasenvironment. Instead of assuming the sticking coefficient values asunity, the experimentally found value of sticking coefficient

(aAl/aTi ¼ 0.4) for nitrogen on Al/Ti was used for the theoreticalcalculations for the composition prediction of the metallic consti-tuents of the coatings. The experimental values follow the theo-retical pattern calculated using the relative sticking coefficient ofaAl/aTi ¼ 0.4.

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