a new technique to measure through film thickness fracture toughness

$: A new technique to measure through film thickness fracture toughness$
Thin Solid Films 401(2001) 203–210

0040-6090/01/$ - see front matter� 2001 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž01.01613-3

A new technique to measure through film thickness fracture toughness

Ting Y. Tsui , Young-Chang Joo *a,1 b,

Advanced Micro Devices, One AMD Place, P.O. Box 3453, Sunnyvale, CA 94088-3453, USAa

Seoul National University, School of Materials Science and Engineering, Seoul 151-742, South Koreab

Received 25 May 2001; received in revised form 28 August 2001; accepted 6 September 2001

Abstract

A new experimental procedure was developed to measure the in plane, through film thickness, fracture toughness of hard thinfilms deposited on soft substrates. Pre-cracks were fabricated in thin film specimens using focused ion beam(FIB) millingtechniques. The crack opening force was generated by means of the indentation sinking-in effect. The effect creates a bendingmoment and tensile stress on the thin film near the pre-cracks and promotes crack extension. The amount of crack tip bluntingprior to the critical failure was characterized by inspecting the crack cross-sections. The fracture toughness of the thin film canbe qualitatively evaluated by using the crack tip opening displacement model(CTOD). The fracture toughness results of twohard thin filmysoft substrate systems are presented; hard nickel phosphorus films on soft aluminum substrates and titaniumyaluminum multilayered films on aluminum substrates. Finite element analysis(FEA) techniques were also used to understand themode of mixity near the crack tip. The results indicated that the crack tip mode of mixity is dominated by the Mode I opening,provided that the indentation location is sufficiently far from the pre-cracks.� 2001 Elsevier Science B.V. All rights reserved.

Keywords: Fracture toughness; Indentation; Thin film; Aluminum; Nickel phosphorus

1. Introduction

There is an increased interest in the mechanicalproperties of materials at very small scales, especiallyin the form of thin films. This is partly the result ofrapid miniaturization in the semi-conductor and themagnetic hard disk technologies. For example, the totalthickness of the magnetic and super-hard thin films ona hard drive is thinner than 100 nm. The gate oxide inintegrated circuit transistors is even thinner, less than 30A. The mechanical properties of these thin films are˚often related to the life time reliability of the finalproducts. In recent years, many experimental techniqueswere developed to measure mechanical properties ofthin films. For example, the stress and strain propertiesof thin films can be characterized by using bulge tests,

* Corresponding author. Tel.:q82-2-880-8986; fax:q82-2-883-8197.

E-mail address: [email protected](Y.-C. Joo).Present address: Texas Instruments, 13570 North Central Express-1

way, MS 3701, Dallas, TX 75243, USA.

laser-scanning substrate curvature techniques, or X-raydiffraction techniquesw1–4x. The hardness and elasticmodulus of thin films are commonly measured by thenanoindentation methodw5–12x. Non-destructive tech-niques, such as Raman spectroscopyw13–15x, are alsoavailable for researchers to investigate thin film elasticproperties and stress states. These techniques, which canmeasure elastic modulus, strain, stress and hardness, arewell established and commonly used. However, fracturetoughness, one of the most important mechanical prop-erties during engineering design remains difficult tomeasure for thin filmsw16,17x.

Fracture toughness measurements can easily be per-formed on bulk specimensw18x. During a ‘macroscopic’fracture experiment, a pre-crack or pre-cracks withspecified length and geometry are machined on thespecimen prior to loading. Then, the specimen is sub-jected to a specified loading condition, depending onthe information desired, until the sample fails. Thedifficulties of applying the ‘macroscopic’ fracture testingprocedure to thin films are three fold. The first difficulty

204 T.Y. Tsui, Y.-C. Joo / Thin Solid Films 401 (2001) 203–210

Fig. 1. SEM micrograph of a cross-sectioned Knoop indentation in a9-mm-thick hard NiP film on a soft aluminum substrate. The figureshows the materials around the indentation depressed downward eventhough they were not in contact with the indenter, i.e. sink-in effect.

is manufacturing a sharp pre-crack in the thin film witha controlled length and geometry. The second challengeis the introduction of a well-controlled crack extensionforce to the thin film. Finally, the detection of acatastrophic failure in the thin film is a challengebecause it is almost impossible to detect nanometer scalecrack propagation through the film thickness.

We have developed a new experimental procedure tomeasure the near plane strain through film thicknessfracture toughness of hard thin films deposited on softsubstrates. The three difficulties in measuring thin-filmfracture toughness were overcome by using the focusedion beam(FIB) milling techniques to manufacture pre-cracks, indentation sinking-in effect to generate stablecrack extension force, and using the crack tip openingdisplacement(CTOD) model to characterize the metallicthin film fracture toughness. It will be shown that thismethod can measure thin film fracture toughness ofnickel phosphorus(NiP) thin films on aluminum(Al)substrates and six titaniumyaluminum(Ti yAl) bilayeredthin films on Al substrates. The mode of mixity betweenMode I and Mode II during the critical failure was alsoevaluated by using the finite element analysis(FEA)method.

2. Theoretical background

In this work, thin film pre-cracks were manufacturedby using advanced micromachining techniques, specifi-cally focused ion beam(FIB) milling. One of theadvantages of using the FIB milling technique is itsability to target a localized area and produce an extreme-ly sharp pre-crack with tip radius in nanometer scale.Extremely sharp crack tips are required in the thin filmfracture experiment because the plastic zone extendedin front of the crack tip should be small in order toreduce the influence from the substrate. If the crack tipplastic zone extended into the substrate, the results willbe difficult to interpret. During the FIB milling process,Ga ions are used to remove the target materials fromq

the surface. However, implantation effects occur togetherwith the milling process. This creates a damaged zonenear the crack tip and affects the local fracture proper-ties. Fortunately, the thickness of the damaged layerthickness is usually very thin and varies with the kineticenergy of the Ga ions. Susnitzky and Johnsonw19xq

performed a transmission electron microscopy(TEM)study to investigate the ion damaged thickness producedby a commonly used 30 kV Ga ion FIB millingq

procedure on a semiconductor grade silicon substrate.Their results showed that at 68 milling angle, the FIBmilling damaged silicon layer is amorphous with athickness of;280 A. With milling angle at 908, the˚damaged thickness increases to;730 A.˚

The new technique uses the indentation sink-in effect

to produce crack extension force. The sink-in effect hadbeen reported by Tsui et al.w20x and Hay and Pharrw21x on hard thin films deposited on soft substrates. Fig.1 illustrates an example of the sinking-in effect. Thefigure shows a scanning electron micrograph of a FIBcross-sectioned Knoop indentation made in a 9-mm-thick hard NiP film deposited on a soft Al substrate.The specimen was tilted by 528 to reveal the cross-sectioned areas. Fig. 1 shows the materials around theindentation are depressed downward even though theywere not in contact with the indenter(i.e. the sink-ineffect). The sink-in effect creates a bending momentnear the pre-crack produced by the FIB milling andpromotes crack extensions.

In an attempt to estimate the fracture toughness valueof the tested films, we applied the crack tip openingdisplacement model(CTOD) or the strip-yield model.Researchersw22–28x have used the method to estimatethe fracture toughness of bulk materials. The modelssuggested that the fracture toughness of non-hardeningmaterial,K , can be expressed as a function of crackIC

tip blunting immediately before the catastrophic failure,d, yield strength,d , and the elastic modulus,E, of they

material by an equation:

yK s mds EIC y (1)

where m is a dimensionless constant that is approxi-mately 2.0 under the plane strain conditionw27x. Mate-rial parameters on the right side of Eq.(1), elasticmodulus(E) and yield strength(d ) can be measuredy

by using the nanoindentation techniques or the wafercurvature method. The amount of crack tip blunting canbe measured by inspecting the cross-sectioned cracksproduced by the FIB milling method.

205T.Y. Tsui, Y.-C. Joo / Thin Solid Films 401 (2001) 203–210

Table 1Nanoindentation hardness and elastic modulus values of the films and substrates tested in this work

Nickel Titaniumyaluminum Aluminumphosphorous multilayered layer substrate

Hardness(GPa) 7.0 1.0 1.0Elastic modulus(GPa) 160 120 70

Fig. 2. SEM micrograph of a cross-sectioned pre-crack in the NiPfilm. The pre-crack trench is;3.5 mm deep with a tip radius lessthan 10 nm. The groove extended from the crack tip is the ion millingartifact during the cross-sectioning procedure.

Fig. 3. SEM micrograph of a pre-crack in the TiyAl multilayered film.This pre-crack is;1.5 mm wide and;3 mm deep. The tip radius isless than 10 nm.

3. Experimental procedure

Two hard thin film on soft substrate systems werecharacterized in the study; nickel phosphorous(NiP)films on soft aluminum substrates and six bi-layeredtitanium (Ti)yaluminum (Al) thin films on aluminumsubstrates. The 11-mm-thick amorphous NiP thin filmswere deposited on aluminum substrates by electrolessplating at 908C for 2 h. After deposition, the films weremechanically polished to obtain a final thickness of 9mm. The multilayered Al and Ti films were sputterdeposited by using the DC magnetron sputtering tech-niques. The aluminum sputtering power and depositionrate were 7.5 kW and 100 nmymin, respectively. TheTi deposition conditions were at 2.0 kW and 25 nmymin. The Ar base pressure was maintained at 3 mtorrduring the deposition. The thickness values of Al andTi films are 200 nm and 700 nm, respectively. Themechanical properties, such as nanoindentation hardnessand elastic modulus of the NiP and TiyAl films, have

been reported elsewherew20,21x and summarized inTable 1.

Pre-crack trenches in the films were manufacturedusing a FEI 820 dual beam ion milling instrument. Themilling process was carried out using Ga ions operatingq

at 30 kV and 2700 nA for NiP films and 1000 nA forthe multilayered films. An example of a cross-sectionedNiP pre-crack trench is shown in Fig. 2. The pre-crackis ;3.5 mm deep with a tip radius approximately 10nm. This SEM micrograph shows the crack is symmetricnear the crack tip but deviates near the surface. This isdue to the asymmetric energy distribution of the ionbeam being used in this work. However, the fracturetoughness should not be affected by this defect sincethe crack opening stress is concentrated near the cracktip. A faint vertical groove can be found extendingbeneath the crack tip in Fig. 2. This is not an extensionof the crack but an ion-milling artifact produced duringthe cross-sectioning process.

Fig. 3 reveals a pre-crack trench prepared in the TiyAl layered sample by the FIB milling techniques. TheSEM micrograph shows the alternating layers of titani-um (gray layers) and aluminum(dark layers) films.Nanometer scale pores can be observed throughout thefilm. The pre-crack trench is approximately 3mm deep


Fig. 4. Plot of the vacancy concentration as a function of depths inaluminum, titanium, and nickel phosphorus films. The energy ofGa ions modeled is 30 kV.q

Fig. 5. A schematic drawing showing the orientation of the Knoopindentation relative to the pre-crack trench.

and has a tip radius less than 10 nm. The crack tip islocated at the fourth titanium layer beneath the surface.Again, the faint vertical groove extended below thecrack tip is a FIB milling artifact generated during thecross-sectioning procedure.

To understand the degree of Ga ion beam damagedto NiP, Ti and Al films during the pre-crack preparations,Monte Carlo simulations, SRIM 2000w29x, were carriedout. A milling angle at 908 from the surface wasassumed. In each simulation, 50 000 gallium ion impactevents were simulated. The degree of ion damages fortitanium, aluminum and nickel phosphorous, expressedas vacancy concentration, is plotted as a function ofdepths in Fig. 4. The plot shows the maximum iondamage region is located near the surface but taperedoff further into the bulk material. The FIB ion damagedzone thickness for NiP, Ti and Al are 300 A, 400 A and˚ ˚500 A, respectively. It is important to note that the˚fracture toughness measurements should be conductedafter the cracks have been propagated beyond the Gaion damaged zone.

The crack extension force was produced by the sink-in effect from a Knoop indentation made at approxi-mately 20 mm from the pre-crack trench. A sharpdiamond Knoop indenter was used and the load appliedfor Knoop indentation was approximately 2 kg. Thelocation of indenter and pre-crack is schematicallyshown in Fig. 5. A Knoop indenter was chosen in thiswork because the stress state generated near the indenteris closely resemblance to the plane strain condition, i.e.the thin film fracture toughness measured will also beclose to the ideal plane strain valuew20x. Anotheradvantage of the Knoop indentation is the possibility ofgenerating a wide range of bending momentum by asingle indentation due to the unique geometry of theindentation tip. When a Knoop indentation is madeparallel to the pre-crack trench as shown in Fig. 5, the

amount of sink-in and the tensile stress generated at thepre-crack are the greatest at the middle of the indentationand decrease toward the elongated edges of theindentation.

After removal of the indenter, the trenches were cross-sectioned by the FIB milling techniques. The amount ofcrack tip blunting and crack growth at various locationsalong the pre-crack trench was inspected by high reso-lution SEM. By knowing the amount of crack tipblunting prior to the catastrophic crack growth, thefracture toughness of films were estimated by using Eq.(1).

4. Results and discussion

4.1. NiP films on Al substrates

Fig. 6 is a SEM micrograph from the NiP filmyaluminum substrate specimens. It shows the FIB cross-sectioned crack after the removal of the indentationnearby. The indentation edge is;20 mm from thetrench. The materials located near the indentation arebent downward and are stretched in the in-plane direc-tion. When the cracks shown in Figs. 2 and 6 arecompared, it is clear that the crack has grown slightlyfrom ;3.5 to;4.4 mm and the crack tip opening(d)increased to;300 nm. The plane view measurementsalso give a crack tip opening displacement of;300nm. Note the crack tip has propagated significantlyfurther than the FIB ion damaged zone thickness,;30nm, calculated by the Monte Carlo simulations.

As discussed above, the fracture toughness can beestimated by using the crack tip opening displacement


Fig. 6. SEM micrograph of a cross-sectioned crack in NiP film afteran indentation was made at 20mm from the crack. The crack lengthhas grown from;3.5 mm to;4.4 mm and the crack tip opening(d)increased to;300 nm.

Fig. 7. A SEM micrograph showing the locations of the crack tipinspected.

Fig. 8. SEM micrograph of a cross-sectioned crack tip. The inspectionis located at 20mm from the indentation edge. The tip opening is;32 nm.

(CTOD) model provided the amount of crack tip open-ing and the mechanical properties of the material areknown. Hay and Pharrw20x measured the hardness andthe elastic modulus of electroless plated NiP by usingthe depth sensing indentation techniques and reportedvalues of 7 GPa and 160 GPa, respectively. Since thehardness of a material is approximately three times ofits yield strength(d ) w30x, the NiP yield strength isy

estimated as 2.3 GPa. By using Eq.(1), the fracturetoughness of NiP is determined to be 15.0 MPa .ym

The value we calculated by this method is muchsmaller than the bulk nickel toughness value,)100MPa w31x. The compositional difference, NiP vs. Ni,ymas well as the structural difference, thin film vs. bulk,may cause the difference. We were not able to find anyliterature reports with NiP fracture toughness values.The estimated value by this method can also be affectedby the film residual stress and the influences from thesubstrate. Further study is warranted to understand theeffects. However, the new technique can provide a firstorder estimation on the toughness of the film materials.

4.2. TiyAl multilayered film on Al substrate

The crack propagation behaviors of the TiyAl multi-layered film are particularly interesting since titaniumand aluminum have different fracture characteristics.The amount of crack tip blunting should vary with thelocation in the film. The amount of tip blunting forcracks terminated in the aluminum layers should begreater than in titanium because aluminum is a moreductile material.

After a Knoop indentation was made near the pre-crack trench to induce crack growth in the TiyAlmultilayered film, FIB cross-sections and SEM inspec-tions were carried out on the same trench at four

different locations. Fig. 7 shows an overview of thecross-sectioned and inspected locations.

Fig. 8 shows the crack tip located furthest away fromthe indentation,;20 mm. The crack tip is located inthe fourth titanium layer and blunted slightly(d isapprox. 32 nm) when compared with the pre-crackgeometry prior to loading shown in Fig. 3(d-10 nm).The crack length, however, remains the same, i.e. nocrack extension. Fig. 9 shows another cross-sectionedcrack located;17 mm from the edge of the indentation.The crack propagated slightly,;100 nm, but the cracktip remains within the fourth titanium layer. The cracktip blunting is more pronounced,;41 nm, than thecracks illustrated in Figs. 3 and 8. Fig. 10 shows thetrench cross-section located at approximately 14mmfrom the indentation. The figure shows the crack prop-agated through the fourth titanium layer into the alumi-num layer below. The crack tip blunting in Fig. 10 issignificantly greater(d ;76 nm) than that in Ti films


Fig. 9. SEM micrograph of a cross-sectioned crack tip. The inspectionis located at 17mm from the indentation edge. The tip opening is;41 nm. Fig. 10. SEM micrograph of a cross-sectioned crack tip. The inspec-

tion is located at 14mm from the indentation edge. The tip openingis ;75 nm.

Fig. 11. A schematic drawing of the boundary conditions and themodel geometry used in the finite element simulations.

as shown in Fig. 8 and Fig. 9. This suggested that thealuminum film is more ductile than the titanium layer.Note that the exact quantification of fracture toughnessusing Eq. (1) is difficult in the multilayered system.Individual layer thickness is too small to obtain theintrinsic material properties. The amount of tip bluntingcan be affected by the presence of the neighboringlayers that have different mechanical properties. How-ever, the results qualitatively demonstrate the fracturetoughness differences between individual Ti and Alfilms.

5. Finite element analysis

In order to understand the crack tip mode of mixityby the indentation sink-in effect, FEA simulations wereperformed using the software ANSYS(ANSYS Inc).Plain strain elements with eight nodes(Plane 82) andcontact elements(Contact 48) were used. Due to thelimitation of the software, only purely elastic thin filmcan be modeled for the study to illustrate the deforma-tion behaviors near the crack tip. A silicon oxide filmon an aluminum substrate was simulated. The crack tipgeometry of the model is shown in Fig. 11, with a pre-crack of 2.7mm deep and 1.6mm wide. It is located20 mm from the indentation apex. The contacting inter-face between the indenter and the film was modeledwith static friction coefficient of 1. The normal andsticking contact stiffness values are 700 and 7 MNym,respectively. These values are chosen such that excessivecontact element overlapping can be avoided during thesimulations. Dynamic friction is set at the same valueas the static friction coefficient, 1. During the simulation,the oxide thin film is assumed to adhere perfectly to the

aluminum substrate. The indenter is modeled as a wedgewith an apex angle 1308 and a flat tip 1mm across. Themodel consists of 4383 elements and 7529 nodes. Theelastic moduli of the silicon oxide and aluminum filmswere assumed to be 70 GPaw30x. The oxide film wasmodeled as a linearly elastic material while the alumi-num substrate is a perfectly plastic material with yieldstrength at 300 MPa. The Poisson’s ratios of the mate-rials are 0.20 and 0.34, respectively. All of the elasticand plastic properties of the materials are assumed tobe isotropic. Both of the film and the substrate modeled


Fig. 12. Plot of the stress intensity factor as a function of the inden-tation depth for a 5.4-mm-thick silicon oxide film. Note the Mode Iopening dominates the crack tip deformation.

are assumed to be void-free. The Penalty method,Newton–Raphson method, elastic columbic frictionoption, and large deflection option were utilized in thefinite element computations.

Fig. 12 shows the dependence of the Mode I(K )Iand Mode II (K ) stress intensity factor on the inden-II

tation depths for oxide film on aluminum substrate.When the crack begins to grow catastrophically, thecorresponding stress intensity factor is the fracturetoughness,K . It is important to note that the distanceIC

between the indentation edge and the pre-crack trenchreduces with the indentation depth. The rate of reductiondepends on the indenter geometry and the magnitude ofthe sinking-in effect. Fig. 12 shows the crack tipdeformation is dominated by the Mode I openings andthe stress intensity factor increases with indentationdepths. In contrast, the Mode II fracture component issmall within the simulated depths. The Mode II stressintensity factor is near zero for indentation depths lessthan 60% of the film thickness and increases graduallyfor larger depths.

The FEA results suggested that the crack tip openingproduced by the indentation sink-in effect is dominatedby Mode I opening, provided the indentation depth issmall or when the indentation is sufficiently far fromthe pre-crack trench. Further work is needed to under-stand the mode of mixity near the crack tip producedby this technique with different film thickness andmechanical properties.

6. Conclusions

A new experimental procedure has been developed tocharacterize the thin film fracture toughness in thinfilms. FIB milling techniques were used to manufacturepre-cracks in the specimens. The crack extension forcewas introduced by using the indentation sinking-in

effect. The amount of crack tip blunting and crackextension was measured by the means of FIB cross-section method and high resolution scanning electronmicroscopy. The fracture toughness of metallic thin filmscan be estimated by using the CTOD modelwEq. (1)x.The mode of crack tip deformation of the new techniqueis dominated by the Mode I openings provided theindentation depth is sufficiently small or the indentationedge is far from the pre-crack trench.

The fracture toughness of the nickel phosphorus(NiP)film on aluminum substrate was estimated from thismethod and it is approximately 15 MPa . However,ymthis value may be affected by the film residual stress,uncertainty of film’s Young’s modulus and yield strengthvalues, and the measurement error of crack tip openingby FIB. A qualitative comparison of fracture toughnesswas made on the titanium and aluminum multilayeredthin films. Crack tip blunting of aluminum layers wasgreater than titanium, which agrees with the literaturew31x.

Acknowledgements

The authors thank Dr George M. Pharr and Dr EasoP. George of the Oak Ridge National Laboratory fordiscussions on the fracture behaviors of titaniumyalu-minum alloys, and Dr P.Y. Yim for providing the NiPspecimen for this work. They thank Professor WilliamD. Nix of Stanford University for providing equipmentsupports and the multilayered thin film specimens.

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a new technique to measure through film thickness fracture toughness

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