Thin Solid Films 447–448(2004) 462–467

0040-6090/04/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0040-6090Ž03.01125-8

Effect of sputtering target power on microstructure and mechanicalproperties of nanocomposite nc-TiNya-SiN thin filmsx

Sam Zhang*, Deen Sun, Yongqing Fu, Hejun Du

School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

Abstract

Nanocrystalline TiN has been imbedded in amorphous silicon nitride matrix to form a super hard nanocomposite thin film(nc-TiNya-SiN ) via magnetron sputtering. Adjusting Ti and Si N target power ratio altered film composition, size, amount andx 3 4

distribution of the nc-TiN phase. At a Ti target power density of 5.5 W cm , the Ti to Si N target power ratio should be greatery23 4

than unity in order for nc-TiN to form, otherwise, Ti will dissolve in amorphous SiN . The relationship between the film hardnessx

and the crystallite size show Hall–Petch and anti-Hall–Petch relationship. A ‘scratch crack propagation resistance’ parameter, orCPRsL (L yL ), has been proposed to approximate thin film toughness from the critical load data easily obtained from as c1 c2 c1

scratch adhesion test.� 2003 Elsevier B.V. All rights reserved.

Keywords: Nanocomposite; Ti–Si–N film; Microstructure; Hardness; Toughness

1. Introduction

Nanocomposite films have received much attentionrecently because they have improved mechanicalw1–3x,electronic w4,5x and magneticw6x properties owing tothe size effectw7–9x, unattainable from just a layeredcompositew10x. In nano scale, crystallite size togetherwith the crystallite fraction, are extremely important indetermining all these exotic properties. Schiotzw11x putforth a theoretical modeling and predicted that when thegrain size is in the nano scale, the hardness of thematerial increases sharply as grain size reduces and afterit peaks, further reduction in grain size results in adrastic decrease in hardness. Si N based ceramics have3 4

excellent properties of low density, high strength, hard-ness and toughness, wear-corrosion, oxidation, thermalshock resistance, low thermal expansion, self-lubricat-ing, heat insulation, electricity insulation, etc. Hard andtough Si N films thus have wide applications in man-3 4

ufacturing industries. Veprek et al.w12x reported ahardness value of 105 GPa for their nanocomposite nc-TiNya-Si N thin films prepared by plasma assisted3 4

chemical vapor deposition. However, no toughness dataare available to assess the engineering applicability. The

*Corresponding author. Tel.:q65-6791-4400; fax:q65-6791-1859.

E-mail address: msyzhang@ntu.edu.sg(S. Zhang).

present study employed magnetron sputtering depositiontechnique to the study the Ti–Si–N film system, ana-lyzed the effect of target power on forming nanocrystal-line phase and the size effect on hardness. Voevodinand Zabinski’sw13x idea of using lower critical load asa measure of the fracture toughness is modified toinclude the difference between the higher and lowercritical load thus caters to assess thin film’s ‘scratchcrack propagation resistance’. This crack propagationresistance is able to judge the toughness of the film.

2. Experimental details

2.1. Preparation of the films

Nanocomposite nc-TiNya-SiN thin films were pre-x

pared through co-sputtering of a 7.62 cm Ti and Si N3 4

targets in AryN atmosphere using E303A magnetron2

sputtering system(Penta Vaccum, Singapore). Silicon(100) wafers with a diameter of 10.16 cm and thicknessof 450mm were used as substrates. The substrates wereultrasonically rinsed in acetone before deposition. Thesubstrate holder rotated at 15 rev.ymin during the dep-osition for uniformity. The deposition chamber’s basepressure was 1.33=10 Pa, and during deposition they5

gas pressure was 0.67 Pa and gas flow rate was set at30 sccm. The substrate-to-target distance was 100 mm.

463S. Zhang et al. / Thin Solid Films 447 –448 (2004) 462–467

Table 1Magnetron sputtering conditions and resultant film composition, crystallite sizeyfraction and mechanical properties(hardness and adhesion strength)

Code Magnetron sputtering conditions Film elemental composition Crystallite Hardness Critical

Power on 3-inchtarget(W)

Ratio of N to Ar2 (at.%) as revealed by XPS Size Fraction (GPa) load (mN)

Ti Si N3 4 Si Ti N

(nm) (%)

Lc1 Lc2

P1 250 50 1 1.7 50.4 47.9 7.2 3.6 7.6 200 350P2 250 100 1 5.2 43.1 51.7 8.2 6.7 16.6 300 375P3 250 150 1 7.2 38.1 54.7 7.5 7.3 20.7 250 450P4 250 200 1 11.3 35.1 53.6 5.8 7.1 25.8 225 375P5 250 250 1 14.9 29.4 55.7 6.0 10.8 36.8 175 325P6 200 300 0.5 23.6 21.4 55.0 3.2 1.1 16.4 50 200P7 100 300 0.5 32.5 15.7 51.8 0 0 15.5 50 225P8 0 300 0 52.7 0.0 47.3 0 0 15.0 50 250

Deposition was performed at a substrate temperature of450 8C for 2 h. In this study, eight types of Ti–Si–Nfilm were prepared at Ti and Si N target power between3 4

0 and 350 W in argon and nitrogen atmosphere(c.f.Table 1).

2.2. Films characterization

The thickness of the films was determined to be 600nm using a Jeol JSM-5600LV SEM. Composition pro-filing was conducted through X-ray photoelectron spec-troscopy (XPS) analysis using a Kratos-Axisspectrometer with monochromatic Al Ka (1486.71 eV)X-ray radiation after Ar ion etching to remove surfacecontamination. The pressure of the analysis chamberwas lower than 10 Pa, which increased to approxi-y7

mately 5=10 Pa during ion bombardment. Surveyy5

scans were performed in the 1100 eV toy4 eV bindingenergy range at 1 eV step for all samples. Detailedspectra of the core level lines of different elements wererecorded in 0.1 eV steps. For elemental depth etchingprofiling, an ion gun(Kratos MacroBeam) of 4 keVenergy was used at a sputtering rate of 0.05 nmys withhigh purity Ar gas. For TEM studies, the nanocompositefilms were deposited on potassium bromide(KBr)pellets for 20 min, and then the pellets were dissolvedin distilled water to float off the films. The obtainedfilms were examined using a JEOL 200 kV TEM. AnImage Analyzer was used to measure the crystalline sizeand amount from TEM micrographs. The amount of thecrystalline phase, termed crystallite fraction, was esti-mated as the area ratio of the nanocrystalline phase tothe total image area. For each sample, a large number(at least 10) of TEM micrographs were taken and theimages analyzed for the average crystallite fraction. Thesize of the crystallite was measured from the TEMmicrographs using the image analyzer by measuring twoperpendicular dimensions of a crystallite. The averageof these two-dimensional measurements was taken asthe crystallite size. Though this method results inapproximately 10% uncertainties due to contrast prob-

lem, it does give a first-degree approximation plus theadvantage of the visual. Hardness was evaluated usinga Nano II� nano-indenter with a Berkovich indenter.The indentation depth was set at 50 nm(less than onetenth of the film thickness) to avoid substrate effect. Atleast 10 indentations were made on each sample andanalyzed by the Oliver–Pharr method. The load bearingcapacity or scratch adhesion strength was determinedusing Shimadzu SST-101 scanning scratch tester: whilethe X–Y table was moving in theX-direction, it alsomoved side-ways(in the Y-direction) to generate a‘scanning scratch’ effect and thus, greatly increased thecoverage of each scratch. At the same time, an increas-ing normal load was applied continuously to the surfaceof the film through a diamond indenter of 15mm inradius until total failure of the film. In this study, ascratch speed of 2mmys and scanning amplitude of 50mm were used. Five tests were performed for eachsample.

3. Results and discussion

3.1. Chemical composition and mechanical test results

A typical XPS depth profile of the nc-TiNya-SiNx

film is plotted in Fig. 1. There is an inevitable oxygencontamination in the top most layer of the film. Thoughoxygen exists as deep as 75 nm and carbon at approxi-mately 25 nm, their concentrations are very low, whichare expected to render a very insignificant effect on thefilm’s mechanical properties such as nano indentationhardness.(Should there be any influence on hardnessowing to the existence of oxides, the effect would besuch that the measurements would underestimate thehardness because both Ti and Si oxides have lowhardness: 16 GPa for DC magnetron sputtered TiOx

w14x, approximately 10 GPa in the case of reactivecathodic vacuum arc deposited TiOw15x and 8 GP in2

the case of pulsed magnetron sputtered SiOw16x, etc.)2

Silicon content remains almost constant. Whereas nitro-gen and titanium contents slightly increase in the first

464 S. Zhang et al. / Thin Solid Films 447 –448 (2004) 462–467

Fig. 1. XPS depth profile of the nc-TiNya-SiN nanocomposite filmx

deposited by co-sputtering of Ti and Si N under Ar and nitrogen3 4

atmosphere.

10 nm and then remain at constant values. Table 1 alsolists the chemical compositions of all nc-TiNya-SiNx

films obtained from XPS analysis. From the depthprofile (Fig. 1), the oxygen and carbon concentrationsare too low in comparison with Ti, Si and N, and thusignored in composition computation. It can be seen thatN content for all samples is approximately 50"5 at.%,while the film chemistry of Si and Ti varies greatly withthe experimental conditions, mainly target power. Alsolisted in Table 1 are crystallite size, fraction, nanoinden-tation hardness and scratch adhesion results.

3.2. Microstructure

The bright field TEM morphological appearances ofthe nc-TiNya-SiN film are shown in Fig. 2 for threex

different deposition conditions. The silicon content inthe films decrease drastically with relative increase ofTi target power(absolute decrease of Si N target power,3 4

see Table 1). XRD analysis determines that these crys-tallites are TiN. Proof of the crystallites being TiN alsocomes from the analysis of the select area diffraction(SAD) pattern and indexing, as demonstrated in Fig. 3.SAD of the matrix(where there is no crystallite) givesrise, on the one hand, to a diffused pattern typical of anamorphous phase. Usually, the TiN deposited at suchcondition is in crystalline phase. Silicon nitride, how-ever, grows amorphous even at 11008C w17,18x.

In Table 1, the size of the nc-TiN basically decreaseswith silicon content. With reference to the depositioncondition, actually, the Ti target power does not vary(250 W) while Si N target power increases from 50 W3 4

to 250 W (sample P1 through P5) with N to Ar gas2

ratio as unity. Though the presence of a large amountof N may result to a certain degree of Ti target2

poisoning (by forming TiN on Ti target) that wouldcontribute to some lessening of the Ti ion partialpressure, this effect would be the same for P1–P5 sincethey have the same Ti target power and the same N2

flow rate. However, the increasing of Si N target power3 4

inevitably increases the silicon ion’s mobility and partialpressure while reducing the titanium ion’s partial pres-sure, giving rise to more probability of forming SiNx

than TiN crystals. By the same token, the growth ofTiN nano crystals is hindered, resulting in a decrease inthe crystallite size. The sample of Si N target power of3 4

300 W and 350 W(sample P6 through P8) and Titarget power of 200 W down to zero, the Ti ions do nothave enough mobility or energy to form nc-TiN, thusthe films deposited are basically totally amorphous.

3.3. Crystallite fractions and hardness

Fig. 4 is a typical nanoindentation load–displacementprofile. Fig. 5 displays the relationship between filmhardness together with crystallite fraction as a functionof silicon content. The film hardness increases signifi-cantly to approximately 37 GPa at approximately 15at.% Si, corresponding to the maximum crystallite frac-tion. A further increase in silicon brings about drasticdecrease in hardness to approximately 15 GPa which isa common value reported for the Si N film, which3 4

again, corresponds to the reduction in crystallite fraction.Increase of silicon composition comes from the increasein silicon target power(c.f. Table 1). At higher sputter-ing powers of Si N , more ionic N should be available3 4

for the formation of TiN crystallites from more completeionisation of reactive N gas and certain degree of2

dissociation from the Si N compound. Also, higher3 4

target power effectively exerts stronger bombardmenton the growing film thus passing more kinetic energyto the formation and growth of the TiN crystallites.Though this effect applies equally on the SiN matrix,x

it does not show up since it is amorphous. As a result,the nc-TiN crystallite size and fraction increases with Sicontent. The drastic dip in nc-TiN size and fraction from23.6 at.% Si actually comes from the reduction in Titarget power(Table 1) to 200, 100 and 0 W(c.f.,sample P6–P8). At this power ratio, very little or nonc-TiN is formed.

Plotting nanoindentation hardness against TiN crys-tallite size gives rise to Fig. 6. This clearly demonstrates:(1) as crystallite size decreases(going from right to lefton X-axis), film hardness increases drastically(Hall–Petch relationship); (2) the maximum (37 GPa) isapproximately 7-nm crystallite size;(3) further decreas-es in crystallite size brings about a decrease in hardness(the ‘anti-Hall–Petch’ effect); and(4) the hardness tailsoff to the hardness of an amorphous phase as crystallitesize diminishes. This result matches extremely well withSchiotz’s theoretical predictionw11x.

465S. Zhang et al. / Thin Solid Films 447 –448 (2004) 462–467

Fig. 2. TEM micrographs and corresponding electron diffraction patterns for the nc-TiNya-SiN thin films deposited at(a) Ti 250 W and Si Nx 3 4

100 W, (b) Ti 250 W and Si N 250 W and(c) Ti 200 W and Si N 300 W.3 4 3 4

Fig. 3. Selected area diffraction pattern of the crystallite with patternindexing: nano crystalline TiN phase(nc-TiN) is identified.

Fig. 4. Load–displacement curve of nanoindentation of the nc-TiNya-SiNx composite film deposited at Ti target power of 250 W andSi N target power of 250 W(sample P5).3 4

466 S. Zhang et al. / Thin Solid Films 447 –448 (2004) 462–467

Fig. 5. Hardness varying with nc-TiN crystallite fraction and Si con-tent. Before 20 at.% Si, Ti target power was fixed at 250 W. Afterthat, the Ti target power was reduced to 200 W, 100 W and finallyswitched off. See details in Table 1.

Fig. 6. Nano-indentation hardness vs. nc-TiN size in nc-TiNya-SiNx

magnetron sputtered nanocomposite thin film.

3.4. Critical load and toughness

During the scanning scratch test, the minimum loadat which a failure or cracking occurs is called the ‘lowercritical load’ (L ), and the load at which total peel-offc1

occurs is called ‘the higher critical load’(L ). Thec2

lower and higher critical load from the adhesion scratchtest have been summarized in Table 1. The fracturetoughness of thin films are baffling thin film researchersowing to the complexity of the coatingysubstrate sys-tems and lack of convincing test procedure. Voevodinand Zabinskiw13x directly used the lower critical loadfrom a scratch adhesion test to represent thin film’sfracture toughness. First of all, the lower critical load,L , is not ‘fracture toughness’ in its classical meaning,c1

because what the lower critical load represents is atmost a load bearing capacity of the film or the filmysubstrate system or crack initiation load. Compare twohypothetical films A and B with the sameL but filmc1

A’s L is almost the same as itsL , thus film A is ac2 c1

brittle film, while film B has a highL thus even afterc2

crack is initiated the film will not completely fail untilL is reached, therefore it is a tougher film. Directlyc2

using L as a measure of toughness will mistake bothc1

of these vastly different films as having the sametoughness. TheL can be thought of crack initiationc1

resistance: the higher theL the more difficult it is toc1

initiate a crack in the coating. But initiation of crackdoes not necessarily result in failure; what matters ishow long the material can hold after a crack is initiatedbefore a catastrophic failure occurs: the longer it holds(more load it can bear), the tougher one would considerthe film is.

Thus, the toughness of the film should be proportionalto the difference between the higher and the lowercritical load: (L yL ), and it should also be propor-c2 c1

tional to the lower critical loadL because that repre-c1

sents the resistance to initiation of cracks: the higherthe L the more difficult it is to initiate a crack in thec1

film. To differentiate from the traditional fracture tough-ness concept, let us define a new parameter that mayrepresent the resistance of crack initiation and at thesame time propagation in a scratch test, to give a feelof the toughness of the film. If we call this parameter‘scratch crack propagation resistance’, or CPR , then, ass

a first-degree approximation:

CPRsL (L yL ) (1)s c1 c2 c1

Plugging in the relevant data from Table 1 gives riseto Fig. 7. According to Fig. 7, films deposited beforeP5 (or Ti 250 W and Si N varies from 50 to 250 W)3 4

or where nc-TiN forms in the film, have higher CPRs

(best is at P3 or approx. 10 at.%Si) while those overP5 have lower CPR thus is more susceptible to failure.s

This is a fairly good representation of the experimentaloutcome.

4. Conclusion

1. Nanocrystalline TiN has been imbedded in amorphoussilicon nitride matrix to form a super hard nanocom-posite thin film(nc-TiNya-SiN ) via magnetron sput-x

tering in argon and nitrogen atmosphere.2. At a Ti target power density of approximately 5.5 W

cm , the Ti to Si N target power ratio should bey23 4

greater than unity in order for nc-TiN to form in theamorphous SiN matrix. Deviating from this condi-x

tion, TiN crystallites will not form and Ti will dissolvein amorphous SiN .x

3. The relationship between the nc-TiNya-SiN nano-x

composite film hardness and the crystallite size ofnc-TiN matches the Hall–Petch relationship at larger

467S. Zhang et al. / Thin Solid Films 447 –448 (2004) 462–467

Fig. 7. Adhesion crack propagation resistance(CPR) of magnetron sputtered nc-TiNya-SiN nanocomposite thin film deposited at differents x

experiment conditions.

crystallite size and anti-Hall–Petch relationship as thecrystallite size becomes very small(below 6 nm inthis study).

4. A ‘scratch crack propagation resistance’ parameter,or CPRsL (L yL ), has been proposed tos c1 c2 c1

approximate thin film toughness from the lower(L )c1

and higher(L ) critical load data easily obtainablec2

from a scratch adhesion test.

Acknowledgments

The authors are indebted to Nanyang TechnologicalUniversity for AcRF funding RG12y02.

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