Surface and Coatings Technology 122 (1999) 219–224www.elsevier.nl/locate/surfcoat

Residual stress characterization of diamond-like carbon coatings byan X-ray diffraction method

Sam Zhang a,*, Hong Xie a, Xianting Zeng a, Peter Hing ba Gintic Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638705, Singapore

b School of Applied Science, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore

Received 4 February 1999; accepted in revised form 12 June 1999

Abstract

This paper presents residual stress measurements of amorphous diamond-like carbon (DLC) coatings obtained by studyingthe stress conditions of the substrate surface layer immediately adjacent to the coating via X-ray diffraction ( XRD) with a thinfilm attachment. In such a set-up, the incidence angle a at which the primary beam strikes the specimen is fixed at a glancingangle (2° in our experiments) relative to the sample surface while the detector rotates to collect the diffracted X-rays. Theamorphous carbon coatings were deposited on single-crystal silicon wafers and on polycrystalline KBr substrates in an unbalancedmagnetron sputtering system. The effects of substrate material and deposition parameters on the internal stresses of the coatingsare discussed in detail. XRD with thin film attachment provides a new and more precise way to determine the residual stresses inamorphous coatings. Increasing the relative nitrogen flow reduces the compressive stress level of the hydrogenated amorphouscarbon coatings. Under the experimental conditions studied, higher substrate bias power and sputter power densities both increasedthe compressive stress level. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Coatings; Diamond-like carbon; Residual stress; X-ray diffraction

1. Introduction method [12]. In all of these methods, stresses are mea-sured through the measurement of strain, and the strain

Diamond-like carbon (DLC) coatings have a wide is measured by different ‘strain gauges’. In XRD, therange of applications because of their useful properties ‘strain gauge’ is the d-spacing of a series of planes: theincluding high hardness, optical transparency, low residual stresses cause a change of the spacing of crystalcoefficient of friction, chemical inertness and high electri- planes, reflected as the shift of the diffraction peak tocal resistivity. Residual stresses are inevitably introduced higher or lower angle depending upon the nature of thein the coating during the deposition process. Control of stress (compressive or tensile). Measuring the peak shiftresidual stresses is very important because highly stressed or the lattice parameter change enables measurementscoatings can show poor adhesion [1]. In a sputtering of residual stresses, as reported by Valvoda and col-process, the residual stresses are generated mainly as a leagues [4], Perry and co-workers [5–8], Rickerby et al.result of the bombardment and differences between the [9] and Fischer and Oettel [10], in the analysis of TiN,thermal and elastic properties of the coating and the ZrN and other crystalline thin films. For amorphoussubstrate. Residual stresses in hard coatings affect their coatings, however, because there are no sharp diffractionadhesion strength, microhardness and wear resistance peaks, investigations of residual stresses are usually[2,3]. conducted by using the curvature method [2,13].

Residual stresses can be measured in a number of Recently, Kondrashov and colleagues [14] used theways: X-ray diffraction ( XRD) [4–10], acoustic-wave two-crystal method to determine the curvature (and thusdetection [11], curvature measurement by a laser profi- the residual stresses) of an amorphous DLC-coatedlometer [3] and the electrical resistance or capacitance sample by comparing the substrate’s diffraction peak

with that of the reference substrate. In this paper, wepresent stress measurements of amorphous DLC coat-* Corresponding author. Tel.: +65-793-8577; fax: +65-792-2779.

E-mail address: szhang@gintic.gov.sg (S. Zhang) ings via studying the surface layer stress conditions of

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved.PII: S0257-8972 ( 99 ) 00298-4

220 S. Zhang et al. / Surface and Coatings Technology 122 (1999) 219–224

Fig. 1. Reflection geometry of XRD with thin film attachment. In measurements, the incidence angle a is fixed at a small value (up to 9°).

the substrate using XRD with a thin film attachment. form Talysurf series). Raman spectra of the as-depositedAlthough use of XRD to study the stresses of crystalline DLC coatings were obtained with a Rennishawbulk materials or crystalline films is an established Ramanscope with He–Ne laser radiation of 632.8 nm astechnique, measurement of the residual stresses of amor- the excitation source.phous coatings with the XRD method is a new venture.The amorphous carbon coatings were deposited onsingle-crystal silicon wafers and on polycrystalline KBr 2.3. Residual stress determinationsubstrates. The effects of substrate material and depos-ition parameters on the internal stresses of the coatings XRD spectra were taken using a Rigaku X-rayare discussed in detail. diffractometer (RINT 2000 series, model D/max-2200)

with a thin film attachment. A Cu Ka1 X-ray sourcewas used at 30 kV and 40 mA. The XRD diffraction

2. Experimental geometry of the thin film attachment is shown schemati-cally in Fig. 1. The XRD instrumental error is 0.001°

2.1. Coating deposition 2h. The incidence angle a, which can be set from 0.1°to 10°, was fixed at 2° during scanning. For coatings of

Diamond-like carbon coatings about 1 mm in thick- constant thickness, decreasing the incidence angle aness containing hydrogen and nitrogen (a-C:H and increases the diffracted X-ray intensity, K, significantlya-C:N) were prepared on single-crystal silicon wafers (cf. Fig. 2).(20 mm×20 mm) and on KBr polycrystalline pellets

In the commonly used Bragg–Brentano method,(B13 mm) pressed from KBr powders. The depositions

which operates in the h–2h scan mode, the residualwere carried out in an unbalanced magnetron sputteringsystem by sputtering of solid graphite targets in anargon plus hydrogen and/or nitrogen atmosphere. Thebase pressure in the sputtering chamber was below6.5×10−3 Pa, and the working pressure was 1.3 Paduring deposition. The substrates were placed in a rotarysample holder facing the target about 85 mm above thesample. Radio-frequency (RF) bias was applied duringthe deposition. The bias, target current and the ratio ofgas flow rate of H2 to N2 in deposition were varied andthe influence on the residual stresses assessed.

2.2. Coating characterization

A Jeol JEM 5410 scanning electron microscope(SEM) was used to observe the cross-section of theDLC coatings to determine the microstructure andgrowth mechanism. Coating thickness was determinedby measuring the coating step produced by masking Fig. 2. With decreasing incidence angle a, the X-ray diffraction inten-

sity increases significantly [15].using a laser stylus profilometer (Rank Taylor Hobson

221S. Zhang et al. / Surface and Coatings Technology 122 (1999) 219–224

amorphous hump rather than sharp peaks are presentin the diffraction spectra. Hence, direct measurement ofthe residual stresses in the amorphous coating itself isdifficult. However, one can always measure peak posi-tion and the d-spacing change in the substrate adjacentto the coating/substrate interface. As the affected depthis usually shallow in the case of thin coatings, thetraditional XRD method fails to detect the change.XRD with a thin film attachment, however, makes suchdetection possible since the X-ray penetration depth isvery small, sometimes no more than a few hundredangstroms, depending on the incidence angle and theabsorption constant of the materials, etc. For instance,in the case of TiN coatings [10], as the incidence angle

Fig. 3. Orientation of the diffraction plane to the sample surface. varies from 2 to 10°, the X-ray penetration depth variesfrom 0.59 to 1.72 mm, as obtained through non-destruc-

stress is calculated quantitatively via [16]: tive measurements under Cu Ka1 radiation. The inci-dence angle a at which the primary beam strikes thespecimen can be adjusted to change the irradiations=−

E

n Adn−d0

d0B, (1)

depth. Thus, this method was employed in this study.In our experiment, the incidence angle a is fixed at 2°where E, n, dn and d0 are, respectively, the Young’sto maximize the diffraction intensity and minimize themodulus, Poisson’s ratio, d-spacing of the diffractioninterference from the bulk.plane parallel to the surface of the coating under stress

and the d-spacing of the same series of planes in theabsence of stresses.

3. Results and discussionIn XRD using the thin film attachment, however, theincidence angle is fixed at a glancing angle relative to

3.1. Cross-sectional image and XRD spectrathe sample surface while the detector rotates to collectthe diffracted X-ray. Thus for a single-crystal substrate,

3.1.1. Cross-section image of the DLC Coatingssilicon (100) in this case, the diffracted planes will notFig. 4 is a typical SEM cross-sectional image of thenecessarily be parallel to the sample surface; i.e., they

amorphous carbon coating under study. As can be seenwill be at angle h−a. To account for this effect, the d-from the micrograph, the coating is about 1 mm thick.spacing in Eq. (1) is replaced by l defined as:This is in good agreement with the thickness measuredby the surface profilometer. Fig. 5 is a typical cross-l=

dhkl

cos(h−a), (2)

sectional view of the DLC coating on the KBr substrate.In this case, ‘column-like’ growth is observed even

where dhkl

is the d-spacing for (hkl ) planes, h and a are though the coating is amorphous.the diffraction angle and the incidence angle (Fig. 3).

Rewriting Eq. (1) gives:

s=E

nDl, (3)

where Dl=(l−l0)/l0; l represents the distance of thestressed hkl plane in the direction normal to the samplesurface and l0 is that for the same unstressed hkl plane.In data treatment, since the actual thickness slightlydeviated from the target of 1 mm, the stress obtainedfrom Eq. (3) is then normalized against coating thicknessfor a fairer comparison.

Note that stress measurement by the XRD methodis achieved by measurement of the change in d-spacingcharacteristically revealed as changes in diffraction peakposition. In other words, the samples under study must Fig. 4. SEM cross-sectional image of a DLC coating on a silicon wafer.be crystalline (to give a sharp diffraction peak). Since The coating thickness shown agrees well with the thickness measured

using the laser surface profilometer.DLC coatings are basically amorphous in nature, an

222 S. Zhang et al. / Surface and Coatings Technology 122 (1999) 219–224

Fig. 8. XRD difraction peaks of the KBr polycrystalline substrate atthe immediate vicinity of the a-C coatings at different incidence anglesfrom 0.2° to 0°.

Fig. 5. Typical cross-sectional micrograph of the DLC coating on a is an obvious peak shift towards higher 2h with increas-KBr substrate. ing deposition target power density. To be sure that the

peak shift was not from the bombardment during sputter3.1.2. XRD spectra cleaning of the silicon surface layer, XRD spectra before

The XRD spectrum of the Si(100) substrate before and after sputter cleaning were compared. Under thedeposition is shown in Fig. 6. The main diffraction peaks sputter cleaning conditions used, no peak shift wasare centred around 2h=22°, 55° and 77°. observed due to sputter cleaning. The temperature was

After deposition of the coating, the diffraction below 200°C during deposition. Although thermal mis-spectrum centred around the 55° peak was collected match is one of the contributing factors, it should be aagain under the same conditions and is shown in Fig. 7. minor one because of the low temperature involved.The 55° peak (311) is chosen because of the higher Also, it was assumed that there are no significant changeintensity and lack of amorphous interference from the in refractive index of the substrate before and aftercoating. It can be seen in Fig. 7 that, after coating, there coating. Thus, the peak shift was attributed to the stress

due to coating deposition.The shift D2h shown in the plot is for the deposition

at higher target power density. This D2h reflects thechange of the d-spacing of the hkl plane at 2h. Thischange is used to calculate the residual stresses usingEq. (3). For polycrystalline KBr, no apparent changeof peak position is observed with increasing incidenceangle from 0.2° to 9°, as shown in Fig. 8. Since thesignal taken is from the substrate, not the coating (thecoating was amorphous), the diffraction intensity alsoincreases with increasing incidence angle. It was foundat different incidence angles that there is a slight decrease

Fig. 6. XRD spectrum of the Si(100) substrate before deposition ofof peak position: for (100) single-crystal silicon, 2ha-C at a glancing angle of 2°.decreases by about 0.8° while the incidence angle variesfrom 2° to 9°. Therefore, to ensure a fair comparison,the samples should be irradiated at the same incidenceangle before and after coating. In our experiments withsingle-crystal silicon, a fixed incidence angle of 2° wasused. In the case of the KBr substrate, an incidenceangle of 0.2° was used because the diffraction intensityfrom the polycrystalline surface was strong enough foraccurate analysis at such a low incidence angle.

Fig. 9 compares the spectra for KBr polycrystallinediffracted at the same incidence angle of 0.2° before andafter coating. Aside from the obvious peak shift aftercoating deposition, it is observed that the intensity ofFig. 7. XRD spectra from the specimen of silicon wafer coated with

DLC coating. the peaks at higher 2h angles also increases after coating.

223S. Zhang et al. / Surface and Coatings Technology 122 (1999) 219–224

Fig. 9. XRD spectra of the KBr polycrystalline substrate before and Fig. 11. Variations of residual stress of the DLC thin coatings withafter deposition of the a-C coating, irradiated at 0.2° incidence angle. sputtering power at different substrate biases.

From Fig. 10, the compressive stresses decreases byMoreover, the relative intensity of the peaks is also200% when the relative flow rate of nitrogen gas ischanged: before coating, I3 is higher than I2, but afterincreased 100%. Reduction of stresses in a-C:N coatingscoating I2 became stronger than I3. The change in peakas a result of nitrogen incorporation has been reportedshape was also observed for the silicon substrate after[3,17–19]. Angus and Wang attributed the stress reduc-coating deposition (Fig. 7). Although the mechanism istion to over-constraining [18] of DLC. According tonot clear yet, this may have resulted from the stressthis model, Franceschini et al. [19] further attributeddistribution and its effect on certain planes.the decrease of internal stresses to a reduction of theaverage coordination number and of the over-constrain-3.2. Influence of process parameters on residual stressesing in the a-C:N films as a result of the replacement ofC–H with N–H bonds. With increasing nitrogen, theFig. 10 shows the measured residual stresses as afraction of N–H bonds is increased while the unboundfunction of gas flow ratio. Just as stress measurement(dangling) hydrogen content is decreased. The increasedthrough measurement of curvature changes, the mea-fraction of N–H bonds contributes to the reduction ofsured stress should be a good representation of the stressthe stresses according to the over-constraining model,in the coating. It can be clearly seen that with thewhile a further decrease of the stresses is caused by therelative increment of nitrogen flow, the residual stressreduced amount of unbound hydrogen.decreases appreciably. This result agrees with that

The residual stresses resulting from the growth ofreported by Grill and Patel [3] and Torng et al. [17].sputtered films originates primarily from the specificThe influence of the substrate bias and sputtering powerenergy transfer into the substrate and the growing films.density on the residual stress is demonstrated in Fig. 11.The applied bias power thus plays a big role. IncreasingWith increased sputtering power, the residual stressthe bombardment leads to an increase in the compressivelevels increased (becoming more negative). Comparingstress. This is demonstrated in Fig. 11 which shows that,the data obtained under different substrate biases, higherwith increasing bias power, the compressive residualbias resulted in higher stress level (compare the solidstresses is increased. In our earlier paper [20] it wasand the open circles).reported that increasing bias would result in an increasein residual stresses and that, in turn, would decrease theadhesion. Fig. 11 provides direct stress measurementconfirmation of this. Also seen from Fig. 11 is the effectof sputtering power: at the same bias power, highersputtering power results in greater stresses. The increasein target power density effectively increases the ionbombardment and thus leads to the development ofhigher compressive stresses. A similar trend has alsobeen reported in the case of crystalline TiN coatings[6,10]. In a certain sputtering power range, this may bebest understood by relating the deposition ion energy(E ) with the target power density (Dw) together withbias voltage (Vb) and chamber pressure ( p) through:

Fig. 10. The decrease of compressive residual stress in the DLC thincoatings with increasing relative nitrogen flow. E3DwVb/p1/2 . (4)

224 S. Zhang et al. / Surface and Coatings Technology 122 (1999) 219–224

Increasing the chamber pressure decreases the mean free ing is thin and the sample size is small. Increasing therelative nitrogen flow reduces the compressive stresspath of the ions travelling from the target to the sample

surface where deposition is taking place. Therefore, level of the hydrogenated amorphous carbon coatings.The compressive stress level increases with increasingdecreasing the chamber pressure promotes the collision

of ions at the substrate, which leads to higher residual substrate bias power and the sputtering power density.stress and spontaneous peeling. The same reasoningapplies when the bias power is increased at fixed chamberpressure: increasing bias imparts more kinetic energy to Acknowledgementthe ions, thus promoting residual stresses. Generallyspeaking, below a certain threshold, increasing the target Funding of this work came from the Gintic upstream

project U96-P-059 supported by the National Sciencepower density effectively increases the kinetic energy ofthe sputtered ion. This results in an increase in interfacial and Technology Board, Singapore.stresses due to energetic collision and impingement atthe interface, thus promoting residual stresses.

In crystalline coatings, the three main contributors References[10] to the residual stresses are: (1) thermal expansiondifferences between the coating and the substrate, (2) [1] S.J. Bull, Diamond Relat. Mater. 4 (1995) 827–836.

[2] P.L. Crouse, Diamond Relat. Mater. 2 (1993) 885–889.epitaxial or structural misfit between the coating and[3] A. Grill, V. Patel, Diamond Relat. Mater. 2 (1993) 1519–1524.the substrate, and (3) growth-induced stresses. In amor-[4] V. Valvoda, R. Kuzel, R. Cerny et al., Thin Solid Films 193/194

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167–174.contributors are thermal mismatch and growth, which[6 ] A.J. Perry, M. Jagner, W.D. Sproul, P.J. Rudnik, Surf. Coat.includes sputtering bombardment and atomic arrange-

Technol. 39/40 (1989) 387–395.ment and rearrangement. Therefore, the residual stress[7] A.J. Perry, M. Jagner, W.D. Sproul, P.J. Rudnik, Surf. Coat.

level in amorphous coatings is expected to be lower than Technol. 42 (1990) 49–68.that in crystalline films. This is indeed the case. For [8] A.J. Perry, M. Jagner, P.F. Woerner, Surf. Coat. Technol. 43/44

(1990) 234–244.example, the residual stresses in crystalline TiN, ZrN[9] D.S. Rickerby, S.J. Bull, A.M. Jones, F.L. Cullen, B.A. Bellamy,and HfN are −4 to −9 GPa [5] or −2 to −7 GPa for

Surf. Coat. Technol. 39/40 (1989) 397–408.crystalline TiN [10]; for amorphous carbon coatings,[10] K. Fischer, H. Oettel, Mater. Sci. Forum 228–231 (1996) 301–306.

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kov, A.M. Baranov, Diamond Relat. Mater. 6 (1997) 1784–1788.method can be more precise because of the higher[15] RINT2000 Thin Film Attachment Instruction Manual, Rigaku

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Addison-Wesley Publishing Company, Inc., Reading, MA,due to the original curvature of the silicon wafer, etc. [3].1978, p. 454.

[17] C.J. Torng, J.M. Sivertsen, J.H. Judy, C. Chang, J. Mater. Res.5 (1990) 2490.

4. Conclusions [18] J.C. Angus, Y. Wang, in: R.E. Clausing, L.L. Horton (Eds.),Diamond and Related Materials Diamond-like Films and Coat-ings, NATO-ASI Series B: Physics vol. 266, Plenum, New York,XRD with thin film attachment can be used to gaugeNY, 1991, p. 173.the residual stresses at the interface of amorphous

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to the coating/substrate interface. This provides an [20] S. Zhang, H. Xie, Surf. Coat. Technol. 113 (1–2) (1999) 120–125.alternative way to determine the residual stresses in [21] D.C. Yin, N.K. Xu, Z.T. Liu, Y. Han, X.L. Zheng, Surf. Coat.

Technol. 78 (1996) 31–36.amorphous coatings, especially in cases where the coat-