Residual Compressive Stress of Diamond-like Carbon Films : Control and Usage2007. 4. 12.

krlee@kist.re.kr , 2007. 4. 13.

Outline

Ar background Si addition W addition Perspective

BucklingInterfacial Toughness Elastic Property

Applications of DLC

What is DLC ?Amorphous Solid Carbon FilmMixture of sp1, sp2 and sp3 Hybridized BondsHigh Content of Hydrogen (20-60%)

SynonymsDiamond-like Carbon(Hydrogenated) amorphous carbon (a-C:H)i-CarbonTetrahedral Amorphous Carbon

Bond Structure of Carbon1S2 2S22P2

2-D Analogy of Structurea-C:Hta-C

Properties of Solid Carbon

Deposition MethodsEnergyIon SourceCold Substrate

Example : Filtered Vacuum Arc

Residual Compressive Stress of DLC FilmFilm Deposition

Applications of DLC

Structure and Mechanical PropertiesHardness3-D interlink of the atomic bond network

Residual StressDistortion of bond angle and length

Both are dependent on the degree of 3-D interlinks.

Hardness and Residual Stress

Hardness and Residual Stress

Multilayer (S. Anders et al )Stress is not an interpolation between the data for high and low bias only, but is considerably reduced for the multilayer in comparison to single layers. S. Anders et al., MRS Proc. 383 (1995) 453.

Post Annealing (T. A. Friedman et al )Annealing at 600oC for 1-2 min could reduce the residual stress down to 0.2 GPa, without significant structural change.T. A. Friedmann et al., Appl. Phys. Lett. 71 (1997) 3820.

B addition (M. Chhowalla et al )Residual stress of B added ta-C remains within 1 - 3 GPa for all sp3 fraction. M. Chhowalla et al., Appl. Phys. Lett., 69 (1996) 2344.

Outline

Ar background Si addition W addition Perspective

BucklingInterfacial Toughness Elastic Property

Experimental Film deposition Buffer layer depositionAr 8 sccm ( with gun 1 valve), -750 Vb ta-C deposition at GND substrate biasIn various Ar gas pressures in the chamber Analysis Compressive residual stress Hardness nano-indentor Film Composition RBS Atomic structure NEXAFS, ESR T.Y.Kim et al., J. Appl. Phys. 101, 023504 (2007).

Compressive Residual Stress

Hardness & Strain Modulus

Experimental Results & QuestionsAs the Ar background pressure increased, Stress decreasedHardness didnt changed significantly.

Why did this phenomenon happen?Compositional change?Atomic structural change?

RBS film compositionAr 6sccm treated ta-C filmNo Ar in the films Pure Carbon system!!!

NEXAFS sp2/sp3 bonding ratiosp2/sp3 bonding ratio was not changed at the different process condition.

ESR Defect Density

Stress vs. Defect Density

Decrease in Defect DensityStructural relaxation of distorted sp2 clusters bonding/clusters supports the paramagnetic spin density measurement

Energy Distribution with Ar Background

Ar Massage

Energy DispersionEnergyPopulation

MD SimulationDeposition MethodBrenner potential for carbon atom Substrate Diamond substrate (6a0x5a0x6a0)1512 AtomsIncident atomReference energy = 75 eVDispersion methodGaussian distribution (=0 ~ 10)Density of a-C structure~ 3.14 [g/cm3]Fixed LayerDynamics Layer54.4 A

Residual Compressive StressMD Simulation with Energy Dispersionta-C film deposited by FVAwith Ar background gas

Radial Distribution Function2.02~2.17

sp3 Ratio & Density3.140.03 [g/cm3]53.71.7 [%]

Outline

Ar background Si addition W addition Perspective

BucklingInterfacial Toughness Elastic Property

Si Incorporated ta-CC.-S. Lee et al, Diam. Rel. Mater., 11 (2002) 198-203 S.-H. Lee et al, to be Submitted (2005)

Mechanical Properties

Molecular Dynamics SimulationLennard-Jones: Inert GasEmbedded Atom Method : MetalsMany Body Potential : Si, CInteratomic Potentials

Molecular Dynamics SimulationBrenner force field for C-C bondsTersoff force field for C-Si and Si-Si bondsDiamond substrate : 6a0 x 4.75a0 x 6a01,368 atoms with 72 atoms per layerDeposition Total 2,000 atomsIncident Kinetic Energy : 75 eV for both C and SiSi concentration : 0.5 % ~ 20 %

Fixed LayerFullyRelaxedLayerDeposited atomscreated on this plane

Snapshots after Deposition0.0 % 0.5 % 1.0 % 2.0 % 3.0 % 5.0 % 10.0 % 20.0 %

Distribution of Si Atom in ta-C Film0.0 % 0.5 % 1.0 % 2.0 % 3.0 % 5.0 % : Silicon atom: Carbon atom: Substrate

Residual Compressive StressExperiment : C.-S. Lee et al, Diam. Rel. Mater., 11, 198 (2002).

Atomic Bond StructureExperiment : C.-S. Lee et al, Diam. Rel. Mater., 11 (2002) 198-203 Raman G-peak PositionMD Simulation

Radial Distribution of Pure a-C and Diamond

Radial Distribution Function

Carbon for Satellite Peak

Bond Angle Distribution109.5120.0

Outline

Ar background Si addition W addition Perspective

BucklingInterfacial Toughness Elastic Property

W-DLC by Hybrid Ion Beam Deposition Sputter gun: Third elements addition to DLC (W, Ti, Si ); Ion gun: Easy controlling the ion bombardment energy with high ion flux. Wn+H+, Cm+

Composition of the Deposted Film CSi in substrateArW

Stress & Mechanical properties 213 GPa17015 GPa

TEM MicrostructuresW atoms are dissolved in a-C:H matrix.Nano-crystalline -W2C phases evolve. -W2CAmorphous to crystalline WC1-x transition occurs.

Raman & EELS Spectra

TEM MicrostructuresW atoms are dissolved in a-C:H matrix.Nano-crystalline -W2C phases evolve. -W2CAmorphous to crystalline WC1-x transition occurs.

Role of W atoms- ab initio calculationW atoms dissolved in amorphous carbon matrix played a role of a pivot site where the atomic bond distortion could occur without inducing a significant increase in elastic energy, causing the stress reduction.

Other Candidate Elements

SummaryResidual stress reduction by third element additionSmall amount addition of Si and W is effective.

Ab-initio calculation and MD simulation using a hybrid force field of Brenner and Tersoff type force fieldsSmall amount of Si incorporation in a-C network effectively relaxes the distorted bonds. W atoms dissolved in a-C matrix play a role of pivot site where the atomic bond distortion can occur without inducing a significant increase in elastic energy. Relax Bond Distortion while Keeping 3-D Interlinks

Outline

Ar background gas Si addition W addition Perspective

BucklingInterfacial Toughness Elastic Property

Telephone Cord Buckling

Buckling Configurations

Delamination of Floor Paint Bottom of parking lot of a chinese restaurant in Vancouver (2004)

Off-Piste Run in Hoghfgen

What can we do with this phenomenon?

Quantitative Analysis K.-R. Lee et al , Diam. Rel. Mater., 2 (1993) 218.

What can we do with this phenomenon? Can be a useful tool to estimate the mechanical properties of thin films and the fundamental interface toughness (adhesion)

What can we do with this phenomenon? For Isotropic Thin Films

Measurement of Residual StressCurvature (R)DsDf

What can we do with this phenomenon? For Isotropic Thin Films

DLC Bridges by Micro Fabrication DLC film Deposition ( on SiO2 ) DLC PatterningSiO2 Isotropic Wet Etching

Wet CleaningStrain Estimation

Microstructure of DLC Bridges

Strain of the Buckled Thin FilmsZX2A0

Effect of Bridge Length60mm

DLC Bridges100 V400 V550 V250 V

Biaxial Elastic ModulusBridge Method

DLC film Deposition Cleavage along [011] Direction Si Etching (by KOH Solution) Wet CleaningStrain MeasurementPreparation of Free Overhang

Free Overhang MethodStrain of the free overhangBiaxial elastic modulus

A0 / of Free-hang at 546 nm

Effect of Etching Deptht=546 nmt=55 nm

Elastic Modulus for Various Ion Energies

Nanoindentation t>1.0

Advantages of This MethodCompletely Exclude the Substrate EffectCan Be Used for Very Very Thin Films

Nano-indentationThe elastic strain field >> the plastic strain field Substrate Effect is Significant.Substrate

Substrate Effect on the Measurement

Advantages of This MethodCompletely Exclude the Substrate EffectCan Be Used for Very Thin Films

Elastic Modulus of Very Thin Filmsa-C:H, C6H6 -400VJ.-W. Chung et al, Diam.Rel. Mater. 10 (2001) 2069.ta-C (Ground)

Biaxial Elastic Modulus 20233166100

Structural Evolution of DLC FilmsJ.-W. Chung et al, Diam.Rel. Mater., 11, 1441 (2002).

Residual Stress of ta-C film

Biaxial Elastic Modulus of ta-C film

What can we do with this phenomenon? Can be a useful tool to estimate the mechanical properties of thin films and the fundamental interface toughness (adhesion)

Fundamental Adhesionl

Fundamental Adhesion

Delamination of Patterned Substrate

Conclusions Can be a useful tool to estimate the mechanical properties of thin films and the fundamental interface toughness (adhesion)

This result is the composition of the deposited films measured by RBS. The silicon concentration in the film could be controlled in a systematic way by changing the flow rate of the Ar sputtering gas.As shown in this figure, silicon concentration was strongly dependent on the Ar flow rate.When the Ar flow rate was less than 9 sccm, we could not obtain the Si incorporated ta-C film due to unstable ignition of the magnetron sputter source. However, as the Ar flow rate increased from 9 to 12 sccm, the Si concentration increased from 0.5 to 2.5 at.%. When the Ar flow rate was higher than 12 sccm, the significant increase in Si concentration was observed with increasing the Ar flow rate. When Ar flow rate was 18 sccm, the Si concentration of the film was 85 at.%. In all samples, small amount of oxygen was also incorporated with Si, which seems to be due to the surface oxide layer of the sputter target.However, the ratio of oxygen to silicon was less than 0.1 in most cases. The effect of oxygen on the structure of the film was assumed to be negligible in the present work.Molecular dynamic simulation uses interatomic potential to calculate the interatomic force. Using newtons second law, we can simulate the time evolution of each atoms position and velocity. Hence, the success of the MD simulation is totally dependent on the interatomic potential. In this work, we used embedded atom mothod potential of Co-Co, Al-Al and Co-Al atoms. In order to check the validity of the potential, we calculated the physical properties of Co, Al and CoAl B2 structure. These figures shows the Si atoms only.We can confirm that Si atoms are well dispersed in the amorphous carbon films

First we investigated the change of bond distances using the radial distribution function analysis.The left figure shows the radial distribution of pure ta-C film against the distance.The right figure shows the bond distance of sp3 tetrahedral bonding system.These are the equilibrium 1st & 2nd nn distance in sp3 structure.The distance of the first nearest neighbor has the value below 1.54 A of sp3.The C-C 2nd nearest neighbor atoms sits at 2.54 A.

In the radial distribution of pure ta-C film obtained by the present MD simulation,we can observe the down shift of second nearest neighbor peak below 2.54 A. This is one reason for the observed compressive stress of ta-C films.

The other reason is related to the peak that is found near 2.11 A. We named this peak as the satellite peak.The satellite peak might be treated as a kinetically generated state because this peak was disappeared when annealed at high temperature.Since the satellite peak distorted the atomic structure of the ta-C film, the peak is regarded as the other reason for the observed compressive stress.

In the viewpoint of bond distance, the residual compressive stress of pure ta-C film is originated from the downshift of second nearest neighbor and the existence of the satellite peak.Because these are the indication of the change of the bond length.

When the 0.5 at.% Si atom is incorporated, the second nearest peak moves to 2.54 A, which is the equilibrium 2nd nearest neighbor distance in diamond.The intensity of the satellite peak near 2.11 A found in pure ta-C film was disappeared abruptly with only 0.5 at.% of Si incorporation.The position of the first nearest neighbor peak was not changed largely

Therefore, this behavior means the pure ta-C film has highly residual compressive stress by the compressive bond distance between carbon atoms.and these compressive bond distance are suddenly released to the stable distance by even a very small amount of Si incorporation.

Now, we investigated the bond angle distribution of pure ta-C film and Si incorporated ta-C films.109.5 degree is stable angle of sp3 sites and 120 degree is the stable angle of sp2 sites.Most of the carbon-carbon bond angles is located between 109.5 and 120 degree.

The change of intensity by Si incorporation in the larger bond angles is negligible. In contrast, the intensity in the lower bond angles were suppressed appreciably.

We interested in the distorted bond angles below 109.5 degree, which is expected to be the other main cause of the residual compressive stress in ta-C films.Curves in the yellow area show the pure ta-C film has more distorted bond angles and these distorted bond angle can be release by a small amount of Si incorporation.

Therefore, we can propose that the reason of highly residual compressive stress of ta-C film is the distorted bond angle which has the value below 109.5 degree and these residual compressive stress can be reduced due to the relaxation of distorted bond angles by a small amount of Si incorporation. This figure is the schematic diagram of present work. The carbon and hydrogen ions are come from the ion gun by decomposing the C6H6 precursor source. The W ions are fabricated by the DC magnetron sputter gun by inducing the W target with the sputtering gas Ar. The base pressure is kept at 2.0 10-6 Torr. The substrate bias is - 200 V. Dependent on the Ar fraction, the deposition pressure and power density of target is varied in a small range of 4.2~7.3 W/cm2 and 0.6 ~ 1 10-4 Torr , respectively. The film thickness is controlled around 350 nm by varying the deposition time.

Working gas: Ar, C6H6 (total: 12sccm); Power density of target: 4.2~7.3 W/cm2Subtrate bias: 0 ~ -900V;Thickness: 35050nm

Deriving from the RBS spectra, the W concentration in the film as a function of Ar fraction in the gas mixture is shown in the right figure. With increasing Ar fraction, the W concentration increases monotonically in a manner of linear correlation. However, it is note that W-DLC films cant be obtained as Ar fraction is lower than 55% due to the unstable operation of sputter gun and serious target poisoning. Also beyond the 90%, the films displays the typical characteristics of pure W films with metal gloss. The stress evolution as a function of W concentration is shown in this figure. It is apparent that the W incorporation significantly reduce the residual stress, especially when the W concentration is small. When the W concentration is increased from 0 to 4.2 at.%, the stress abruptly dropped from 3 GPa to 1.5 GPa. Further increasing W concentration to 5.2 at.% causes a temporary increase in the stress. thereafter, the stress monotonically decreases with the increase of W concentration up to 12. 5 at.%. To date, such a curious stress jumping phenomenon has never been reported on the other Me-DLC films. For the ease of next discussion, the position of 4.2 at.% W is defined by a stress jumping point and the total stress reduction area is divided into three regions. High resolution TEM provide us more information on the microstructure of the films as a function of W concentration. During the first region, even the tungsten carbides is formed due to the strong tendency in carbide forming of W, they are still in the dominated amorphous structure, as identified by the diffuse diffraction rings. However, in region 2, the sharpen diffraction ring indicates the emergence of crystalline carbides, even in the manner of weak crystallinity. Entering into the region 3, it is evident that the fraction, grain size and crystallinity of carbides increase with increasing W concentration. the sharpen diffraction rings point at the -W2C. This kind of evolution of tungsten carbides can also be demonstrated by the grazing incidence XRD spectra.

Raman and EELS spectra provide us more information on the atomic bond structure. From the Raman spectra, it is apparent that the peak intensity decreases due to the decrease in total carbon content in the film with W increasing. A similar result is also visible in EELS spectra. By fitting the Raman spectra, it is found that the G-peak position almost keeps constant regardless of the W concentration, which reflects the unvaried sp3 and sp2 fraction in the carbon network. In addition, the EELS spectra also demonstrate the sp2/sp3 ratio changes slightly in the film as a function of W concentration.

In this case, it can be said that the nature of carbon matrix does not change with W incorporation, and which cant account for the stress reduction behavior. There should be other changes of freedom degree of the structure.

High resolution TEM provide us more information on the microstructure of the films as a function of W concentration. During the first region, even the tungsten carbides is formed due to the strong tendency in carbide forming of W, they are still in the dominated amorphous structure, as identified by the diffuse diffraction rings. However, in region 2, the sharpen diffraction ring indicates the emergence of crystalline carbides, even in the manner of weak crystallinity. Entering into the region 3, it is evident that the fraction, grain size and crystallinity of carbides increase with increasing W concentration. the sharpen diffraction rings point at the -W2C. This kind of evolution of tungsten carbides can also be demonstrated by the grazing incidence XRD spectra.

The left two figures are the schematic drawing of the calculation system. The four carbon atoms were arranged as a tetrahedron, with a carbon or w atoms located at the center. For simplicity, unbonded carbon bonds were passivated by hydrogen atoms. The total energy of the system was calculated using one of the bond angles being distorted from the equilibrium angle of tetrahedral bond over the range 90-130 degree. fundamental adhesion . .In contrast to the other measurements method, the present technique has many advantages.The most important advantage is thatthe elastic property of thin film can be measured without the substrate effect, because we can completely exclude the substrate effect by etching process. So we can accurately measure the elastic modulus very thin films, using this method.However, the difficulties of nano-indentation for very thin films / arise from the high sensitivity to the substrate, especially when applying to the system of large difference in mechanical properties between the substrate and the film. The substrate effect is more significant in measuring the elastic modulus than in measuring the hardness The elastic strain field is much wider than the plastic strain field.Hence the elastic behavior during unloading is dominated by the elastic behavior of the substrate.

In contrast to the other measurements method, the present technique has many advantages.The most important advantage is thatthe elastic property of thin film can be measured without the substrate effect, because we can completely exclude the substrate effect by etching process. So we can accurately measure the elastic modulus very thin films, using this method.The free overhang method was successfully employed to measure the biaxial elastic modulus of very thin DLC film.The left figure is the elastic modulus of a-C:H film made by rf-PACVD, and the Right figure is that of ta-C film made by Filtered Vacuum Arc.Here, the a-C:H film is polymeric, but the ta-C film is very hard.Using this method, we could successfully measure the elastic modulus of the film about 33nm thickness.The more important observation is that , in contrast to ta-C films, the elastic modulus of the film decreased when the film thickness was very small, in a-C:H film. In our previous work, we showed that the decrease in elastic modulus of very thin film is not due to the interfacial layer but due to the structural evolution during the initial stage of the film growth.These results show that the mechanical property measured in thick film cannot be always used for very thin film.Therefore, the mechanical properties of the film and the structural evolution during the initial stage of the film growth should be carefully investigated for a specific deposition condition.This Figure shows the dependence of the biaxial elastic modulus on the film thickness A fixed elastic modulus was observed only at red point, hard and dense carbon film depositedIn both case of higher or lower value of V / root P, decreasing the elastic modulus was observed in very thin films.The observed elastic modulus shows that the structural evolution during the initial stage of the film deposition is significant in the films of high content of polymeric or graphitic component.

In polymeric and graphitic films, the elastic behavior of very thin film is similar. The biaxial elastic modulus decreased with decreasing film thickness.But the reason for the decrease of elastic modulus is not the same.

In polymeric film, more polymeric film reduced the elastic modulusIn graphitic film, more graphitic film reduced the elastic modulus

First of all, we analyzed the delaminaton buckling with considering one-dimensional buckling pattern. Buckling geometry can be expressed as shown. And the total work of adheision can be expressed with buckling geometry and the elastic constant.