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Residual stresses and Raman shift relation in anatase TiO2 thin film
Ibrahim A. Alhomoudi a, G. Newaz b,⁎a Mechanical Technology Dept., Hydraulic and Pneumatic Section, Alahsa College of Technology, P.O. Box 804, Hofuf 31982, Saudi Arabiab Mechanical Engineering Dept., Wayne State University, Detroit, Michigan 48202, USA
Anatase TiO2 film (100–1000 nm thick) grown on glass, sapphire (0001), and Si (100) substrates by pulseddc-magnetron reactive sputtering were evaluated for stress and strain analysis using Raman spectroscopyand curvature measurement techniques. The X-ray analysis revealed that films prepared for this study werepurely anatase, and the measurements indicate that the film exhibit that (101) is the preferred growthorientation of the crystallites, especially for the film thicker than 100 nm. Curvature measurements andRaman spectroscopy, with 514.5 nm excitation wavelength, phonon line shift were used for stress analysis. Acomparison between Raman lineshapes and peak shifts yields information on the strain distribution as afunction of film thickness. The measurements of residual stresses for crystalline anatase TiO2 thin filmshowed that all thin film were under compressive stress. A correlation between Raman shifts and themeasured stress from the curvature measurements was established. The behavior of the anatase film on threedifferent substrates shows that the strain in film on glass has a higher value compared to the strain onsapphire and on silicon substrates. The dominant 144 cm−1 Eg mode in anatase TiO2 clearly shifts to a highervalue by 0.45–5.7 cm−1 depending on the type of substrate and film thickness. The measurement of the fullwidth at half maximum values of 0.59–0.80 (2θ°) for the anatase (101) peaks revealed that these values aregreater than anatase powder 0.119 (2θ°) and this exhibits strong crystal anisotropy with thermal expansion.
Anatase TiO2 is most commonly used as a photocatalysis materialfor optical applications [1–6], gas sensors [7–9] and in solar cells[10,11]. It can decompose some pollutions with photocatalytic actionand become important for environmental purification [12,13]; itwas also used for ethanol and methanol sensing properties forbreath analyses [14]. Many different fabrication techniques havebeen developed to prepare TiO2 thin film; however, the processingconditions have been found to strongly influence the structuralproperties of the resulting film. Often the film exhibits strain dueto thermal stresses caused by differential thermal expansion bet-ween the substrate and the film or by the lattice constant mismatchbetween the two. Most of these stresses are either compressive ortensile, so they can eventually result in the failure of devices. It hasbeen recognized that residual compressive stresses may cause filmdelamination from the substrate whereas tensile stress may causesurface cracks in the film [15,16]. The most commonly used methodsto measure the residual stresses in film are substrate curvaturemeasurements [17,18], X-ray diffraction (XRD) [19,20] and Raman
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spectroscopy [21,22]. The XRD and the curvature measurementscan be used directly to determine the average stress in film. It is alsoknown that (first order) zone center Raman phonon lines shift tohigher/lower frequency under compressive/tensile stress. Extractingthe magnitude of residual stress from Raman shifts requires knowl-edge of the relation between these shifts and the lattice strain[15,18,23]. We studied the relation between the Raman shifts and theaverage film strain, which was deduced from experimental measure-ments of substrate curvature induced by stresses resulting from filmgrowth.
Thermal effects provide an important contribution to film stress.Strain develops in a growing film because it is constrained during thedeposition process. Usually this constraint is the bonding to asubstrate. Thermal effects provide important additional contributionsto film strain. The corresponding stresses will in turn strain thesubstrate. This substrate strain generally appears as bending of thesubstrate. The thermal strain is caused by differential thermalexpansion between film and substrate when film is deposited athigh temperature and then cooled. This film will be residuallycompressed when measured at room temperature if αfNαs, with αas the linear expansion coefficient. In this case the substrate shrinksmore than the film. The total stress (σtotal) acting in a film is the resultof three distinct contributions (Eq. (1)). These stresses are as follow:external stress (σext), which is due to possible external loading;thermally-induced stress (σth), which is due to themismatch betweenthe thermal expansion coefficients of both substrate and filmmaterial;
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and intrinsic stress (σin), which is related to the particular coatingmicrostructure and the particular growing process itself [24,25].
σ total = σext + σ th + σ in ð1Þ
The isothermal stress–strain relation, ignoring the effect oftemperature change, is given by Hooke's law, σ=Eε, where the stressis directly proportional to the strain (an approximation limited tosmall strains and certain materials) [26]. E is a symmetric matrix ofthe material stiffness. Anatase film has orthotropic stress–strainrelationships [27] that can be simplified as expressed in Eq. (2), wherethe z-axis is the principal material direction [28,29].
e1e2γ12
8<:
9=;=
S11 S21 0S12 S22 00 0 S66
24
35 σ1
σ2τ12
8<:
9=; ð2Þ
Where S is a symmetric matrix of material compliances (S=E−1),S11=1/ E1, S12=−v12/ E1=−v21/ E2, S22=1/ E2 and S66=1/G12=E12/2(1+v12).
A Raman spectrum is generated when an intense beam of light,usuallygenerated froma laser, is directed onto amaterial. A small fractionof this excitation light is scattered inelastically and shifted to a differentfrequency or wavelength. This change in frequency (Δν) is the frequencyof a particular vibrational mode of the crystalline material (First orderRaman effect) [30–32]. The three natural phases of titanium dioxideTiO2 are anatase, rutile andbrookite. Anatasehas sixRamanactivemodes,A1g (515 cm−1), 2B1g (400 and 519 cm−1), 3Eg (144,197 and 640 cm−1),and rutile has four Raman activemodes,A1g (612 cm−1),B1g (143 cm−1),B2g (826 cm−1), Eg (447 cm−1); both are tetragonal, while brookite isorthorhombic has 36 Raman active modes (9A1g+9B1g+9B2g+9B3g)[15,23,33,34]. In our previous work on anatase TiO2 thin film, we studiedthestructural andmorphological propertiesof amorphous andcrystallineanatase TiO2 thin film [35]. We also studied the operation temperature,film thickness, and substrate effects on the resistance of anatase TiO2
thin filmwhen exposed to CO gas [8]. In this work, our research interestwas to study the influence of the anatase film thickness and sub-strate type, glass, sapphire (0001) andSi (100), for residual stress analysisusing Raman spectroscopy and curvature measurement techniques.This information is important in order to understand the film structure,residual stress in the film, and film performance on different sub-strates. The selection of substrates was because the glass substrate hasan amorphous structure and is a widely used material, where Si (100)substrate selection was based on the most commonly used substrate indifferentdevices.However, the sapphire substratehas the closest thermalexpansion coefficient (9.03×10 −6/°C−1) compared to the anatase TiO2
(10.20×10−6/°C−1), as well as it has high chemical durability and is agood insulator since its band gap (8.8 eV) is much wider than the bandgap of anatase (3.2 eV), thus improving the stability of the sensor.
2. Experimental setup
Anatase TiO2 thin film (100–1000 nm thick) was prepared bymagnetron sputtering in reactive argon/oxygen gas atmosphere onglass, sapphire (0001) and Si (100) substrates. The deposition con-dition was selected from our previous work [35] based on the XRDresults of the best crystalline quality in terms of the maximum inten-sity ratio of the sharp diffraction peak to the broad background.This was carried out with the set of parameters, such as growthpressure (3.0–5.0 mTorr), power (300–500W), and substrate temper-ature (25–400 °C). The anatase thin film was studied for stressanalysis with X-ray diffraction, the Curvature measurement techni-que and Raman spectroscopy measurement. Crystalline structureand crystallite size were determined by means of standard θ/2θXRD scans using a Rigaku-Rotaflex RU2000 diffractometer system
(Geigerflex X-ray Goniometer unit) with Cu source. Vmax=80 kV,Imax=150 mA for a Cu rotating anode. JADE XRD pattern processing(MDI) was used to collect and process the data. The Raman spectrawere recorded in a backscattering geometry using a Renishaw InViaRaman-microscope system and the 514.5 nm excitation wavelengthof an Ar-ion laser, focused on a spot size of the order of ≈3 µm. TheRaman peak positions were obtained by curve-fitting the lineshapedGaussians to the spectra shapes of the raw data obtained from thespectrometer. The curvature (deflection) measurement techniquewas performed using a model 128 intelligent film stress measure-ment system manufactured by Frontier Semiconductor Measure-ments (FSM) Inc., and the Stoney formula was used to derive arelation between the curvature of the system and the stress of thethin film [18].
3. Results and discussions
3.1. X-ray diffraction (XRD)
Film was characterized by XRD, which confirmed the anataseTiO2 structure of the film, and it also showed that the anatase filmis crystalline material. All the observed sharp peaks could be indexedbased on the anatase single-phase structure or assigned to sub-strate reflections, as indicated in Fig. 1(a), (b), and (c). A high peak at25.22 (2θ°) and other very small peaks at 37.75 and 47.72 (2θ°) wereobserved, corresponding to anatase (101), (004) and (200) reflec-tions, respectively. There is no evidence of a rutile phase of TiO2. Theexperimental peak positions were compared with the standard JCPDScard # 71-1169 [36], and the corresponding miller indices wereindexed. The XRD analysis revealed that film prepared for this studywas purely anatase, and the measurements indicate that the filmexhibit that (101) is the preferred growth orientation of the crystal-lites, especially for the film thicker than 100 nm. As was observed, thediffraction patterns of the film deposited on the three substratesshown in Fig. 1(a), (b), and (c) contain the intensity ratio of the (101)peaks that are several times higher than other peaks in addition to theline assigned to the substrates. The film deposed on sapphire (0001)and Si (100) substrates also exhibit (004) orientation but very smallcompared to (101) orientation. Fig. 1(b) shows that the small peakobserved at 37.75 (2θ°) appears somewhat higher because it consistsof two components that are corresponding to the anatase (004) peakand also to the sapphire substrate peak. The broad background on thedata for films on glass corresponds to the substrate, and there is noevidence that the thinner films show an amorphous component.The film on the glass substrate shown in Fig. 1(a) and on the sapphire(0001) substrate shown in Fig. 1(b) are transparent, while thosegrown on the Si (100) substrate shown in Fig. 1(c) present differentcolors related to interface effects.
3.2. Raman spectra
The anatase TiO2 thin film Raman spectra on glass, sapphire(0001) and Si (100) are shown in Fig. 2(a), (b), and (c), respectively,using a wavelength of, λ=514.5 nm. From the figures, the frequen-cies of Raman bands identified as 147.3 (±2.8) cm−1 are assignedto the Eg phononic modes represented by v6. The bands as 392.8(±4.3) cm−1 are assigned to the B1g phononic mode (v4). The bandsas 515.2 (±5.3) cm−1 can be attributed to the A1g phononic mode(v2). The bands as 513.14 (±7.4) cm−1 can be attributed to the B1gphononic mode (v3), and the bands as 628.8 (±10.2) cm−1 can beattributed to the Eg phononic mode (v1), based on the factor groupanalysis. These bands agree well with those in previous studies foranatase powder and single crystals [23,33,34,37,38]. From the factorplane analysis it was observed that both A1g (v3) and B1g (v2) modesinvolve the Ti–O bond stretching normal to the film plane [38]. TheRaman spectroscopy results showed that the thinner film (100 nm) is
Fig. 2. Raman spectra of anatase TiO2 thin film with different thickness as deposited on(a) glass, (b) sapphire (0001), and (c) Si (100) with excitation wave length λ=514 nm.
Fig. 1. The X-ray diffraction patterns of five different thicknesses of anatase TiO2 thinfilm deposited on (a) glass, (b) sapphire (0001), and (c) Si (100) substrates.
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a crystalline material, and the intensity of Raman spectrum of thethin film increases with film thickness. The positions of the majorpeaks are all shifted with respect to the corresponding frequenciesin the bulk material. Fig. 3 shows the Raman spectrum mode shiftsfor anatase TiO2 thin film with different thickness on glass substrateshown in Fig. 3(a), on sapphire (0001) substrate shown in Fig. 3(b)and on Si (100) substrate shown in Fig. 3(c) using the excita-tionwavelength of λ=514.5 nm. The dominant 144 cm−1 Eg mode isshifted towards a higher frequency by an amount that depends onthe substrate as well as on the thickness of the film. Similar shiftshave been previously reported and have been related to the confine-ment effects in nano-structured anatase crystallites [39].
The shifts of the 400 cm−1 B1g and 640 cm−1 E1g modes weretoward lower wave numbers and decrease with film thickness. Asthe broad band near ~517 cm−1 consists of two components thatare not well resolved, the 515 cm−1 A1g and 519 cm−1 B1g modes, thecorresponding shifts indicated in Fig. 3 are unreliable and can beconsidered only as guesses. For the case of the Si (100) substrate evenits location is questionable due to the presence of the strong sub-strate band. If these shifts were caused by the inhomogeneous strainfield induced during the growth caused by lattice mismatches bet-ween substrate and anatase, the shifts should generally decrease with
Fig. 4. Variation of residual stress with anatase TiO2 film thickness on differentsubstrates.
Fig. 3. Raman spectrum shift of anatase TiO2 thin film modes deposited on (a) glass,(b) sapphire (0001), and (c) Si (100) substrate using the excitation wavelength ofλ=514.5 nm.
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increasing film thickness and should be smaller for the spectraobtained with the shorter wavelength excitation. Fig. 3 implies thatthis may indeed be the case. In addition, as shown clearly in Fig. 3(a),(b), and (c) for the 144 cm−1 Eg line, as the shift increases, so does theline broadening, which according to the interpretation given aboveindicates as expected that the strain field within the probed volumeis rather inhomogeneous, but that this inhomogeneity decreases asthe point of probing moves further from the substrate. The four high
wave-number modes (400 B1g, 515 A1g, 519 B1g and 640 Eg modes)have similarity in Raman shifts, where the modes reduce their shiftingfrom a lower energy to a higher energy band gap of the thin filmson glass, sapphire (0001) and Si (100) substrates as shown in Fig. 3(a),(b), and (c), respectively. Another observation is that the high wave-modes have the same slope of shifting, except for the thin film on theSi (100) substrate.
3.3. Curvature measurements
The anatase TiO2 thin film (100–1000 nm) grown on glass, sapphire(0001), and Si (100) substrates have also been used for the curvaturemeasurements of substrates before and after the film deposition, wherethe residual stresses were then determined. The residual film stress(σ) was calculated by using Stoney's formula (Eq. (3)), which is basedon an approximate plate analysis [17],
σ =Esub
6 1− vsubð Þt2subtfil
!1ra
− 1rb
� �MPað Þ ð3Þ
where Esub is the Young's modulus, vsub is the Poisson ratio of thesubstrate, tsub and tfil are the thickness of the substrate and the film,respectively, and ra and rb are the radii of curvature for substratewith and without film. The following have been used with Eq. (3): forglass, Esub=627.58 GPa and vsub=0.2; for sapphire, Esub=370 GPaand vsub=0.2; for Si, Esub=230 GPa and vsub=0.2 [40–43]. In thecurvature measurement, Stoney's equation is justified becausethe film thickness is less than 5% of the substrate thickness. Fig. 4shows the stress as a function of the anatase TiO2 film thickness (100–1000 nm) on glass, sapphire (0001) and Si (100) substrate. Theseresults clearly show that the bi-axial stresses are compressive anddecrease with the increasing film thickness. Further, the magnitude ofresidual stress in the film grown on glass substrate is substantiallylarger than that on sapphire (0001) or Si (100) substrates. The lowerresidual stress in film prepared on sapphire (0001) and Si (100)substrates can be explained based on the closeness of the thermalexpansion coefficient and the lattice match between the substrate andthe anatase film. It is interesting to note that anatase film grown onsapphire (0001) exhibits a stress relaxation over a small thicknessrange compared with the film grown on Si (100) and glass substrates.
Fig. 6. Variation of Δω with film thickness for 144 cm−1 Eg mode of anatase TiO2 thinfilm on different substrates.
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3.4. Residual strain of anatase thin film
The strains for the anatase TiO2 thin film on the three differentsubstrates were calculated based on Eq. (2). The elastic constantsE1=303 GPa, E2=115 GPa and G12=94 GPa for anatase TiO2 weretaken from reference [27], and the Poisson's ratio (vsub) has value asmentioned earlier, where the stress values were obtained fromcurvature measurements. The stress–strain behavior for anataseTiO2 thin film on the three different substrates are shown in Fig. 5,which shows that the calculated data for the film on the threedifferent substrates fit in a straight line. Also, the strain for the thinfilm on glass has a higher value compared to the strain on sapphire(0001) and on Si (100). Furthermore, the stiffness (E=179 GPa) forthe thin film was calculated from the slope of the stress–strain curve.
3.5. Measurement of Raman peak shifts
Quantitative relationships between bi-axial stress and the shift inthe phonon frequencies exist for diamond film [22]. It is known thatthe Raman peak shift (Δω) is proportional to the magnitude ofresidual stress in thin film [21,44]. However, no theoretical workdescribing such relations for tetragonal anatase TiO2 is reported in theliterature; thus, no proportionality constant factor between Δω andthe residual stress in the thin film is known for anatase. The Ramanpeak shift was measured with respect to its position in the stress freesample. Assuming that the diamond result is also correct for anatase,we can write the relation namely,
σ = IAN · Δω MPað Þ ð4Þ
Where IAN (MPa/cm−1) is the proportionality constant. Thevalue of this constant can be calculated based on the stressesobtained from curvature experiments, particularly Raman transition,substrate and excitation wavelength. Δω (cm−1) is the Raman peakshift of the film. The Raman spectra of the anatase TiO2 thin film(100–1000 nm) was measured with 514.5 nm laser excitation, andthe thickness dependent shifts of Eg phonon mode were given above.The dominant 144 cm−1 Eg mode in anatase TiO2 clearly shifted to ahigher value by 0.45–5.7 cm−1 depending on the type of substrateand the film thickness (shown in Fig. 6). It is also shown in Fig. 6 thatΔω decreases with increasing film thickness. Maximum shift wasseen for the film on the glass substrate indicating a higher bi-axialcompressive stress in agreement with the curvature measurements.The shifts of Eg mode clearly show that the bi-axial stress increasesalong the film depth, being larger at the film/substrate interface.
Fig. 5. Stress–strain behavior for anatase TiO2 thin film on three different substrates.
Fig. 7(a), (b), and (c) show a summary of the Raman line shift of144 cm−1 Eg mode obtained with 514.5 nm excitation wavelengthvs. average residual stress values determined from curvature mea-surements on the three different substrates, glass, sapphire (0001)and Si (100), respectively. The Raman peak positions were obtainedby fitting Gaussians to spectra shapes. The value of the proportion-ality constant, IAN (MPa/cm−1), in Eq. (4) was obtained by fittinga straight line as shown in Fig. 7, where the value of IAN was foundto be equal to −937, −337 and −391 (MPa/cm−1) for the anatasefilm on glass, sapphire (0001) and Si (100) substrates as shown inFig. 7(a), (b), and (c), respectively.
The magnitude of the stress in the film changes depending onthe film thickness and substrate. The compressive stresses graduallyincrease with lower film thickness, which means that the stress getshigher closer to the film and substrate interface. In correlation ofthe Raman spectroscopy peaks shift with the curvature measure-ments, Fig. 8 shows the Raman shift vs. strain of the film calculatedbased on Eq. (2). It shows that the shift increases with the strain. Itwas also observed that the Raman band 144 cm−1 broadened withdecreasing strain as shown in Fig. 9. The broadening of these peaksshows that there are contributions to the Raman band shift from theanatase growth oriented at all angles to the deformation axis. Anatasegrowth oriented at 90° to the deformation axis will shift to a higher
Fig. 7. The Raman spectrum shift of 144 cm−1 Eg mode obtainedwith 514.5 nm excitationwavelength vs. average residual stress values determined from curvature measurementson glass, sapphire (0001) and Si (100) substrate.
Fig. 10. The average crystallite sizes of the anatase TiO2 thin film with different filmthickness on sapphire (0001) substrate.
Fig. 8. Raman spectrum shift of 144 cm−1 peak vs. strain for anatase TiO2 thin film onthree different substrates.
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wave-number with strain due to Poisson's contraction of the matrix;compression on the anatase thin film will result in positive shift of144 cm−1 Raman bands.
From XRD analysis, the intensities of the XRD (101) anatase peakwere increased and the widths became broader. In general, the fullwidth at half maximum (FWHM) of the XRD peak corresponds to thecrystal size of the porous materials. When the width was broader, thecrystallites exhibited smaller size. The Scherrer equation (Eq. (5)) wasused to determine the average crystal sizes of the TiO2 nanoparticles.
t =0:9λ
β · cosθð5Þ
Where t is the crystallite size, λ is the X-ray wavelength of theincident radiation (Cu Kα=0.154 nm), β is the broadening ofdiffraction line measured as half of its maximum intensity (FWHM),and θ is the corresponding diffraction angle for anatase TiO2 (101)reflection. The measurements of the FWHM values of anatase (101)peaks were found to be in the range of 0.59–0.76 (2θ°) on the glasssubstrate, 0.6–0.74 (2θ°) on the Si (100) and 0.60–0.80 (2θ°) on thesapphire (0001) substrate. These values of the FWHM for anatase thinfilm on the three different substrates are several times larger than for
Fig. 9. Peak broadening of 144 cm−1 peak vs. strain of anatase TiO2 thin films on threedifferent substrates.
anatase powder (0.395° and 0.119° for filmwith 30 and 200 nm thick),which is assumed to be strain free [45]. Crystallite size values weredetermined using the Scherrer equation (Eq. (5)) to fined the averagecrystal sizes of the anatase TiO2 thin film, assuming that all broad-ening was due to crystallite size. Our XRD analysis of the anatase TiO2
thin film predicted a crystallite size of 10–13 nmon the glass substrate,11–13 nm on the Si (100) substrate and 10–12 nm on the sapphire(0001) substrate. A likely explanation for the slightly low value ofcrystallite size predicted by the XRD analysis resides in the assump-tion that all peak broadening is due to crystallite size effects. Fig. 10shows the average crystallite sizes of the anatase TiO2 thin film withdifferent film thickness on sapphire (0001) substrate, and it clearlystates that the crystallite size is thickness-dependant. It was reportedthat polycrystalline ceramic materials revealed that the effect ofresidual stress among grains caused by thermal expansion anisotropyincreases with increasing grain size, where anatase exhibits strongcrystal anisotropy with thermal expansion [45–48].
The residual stress evaluated by the above-mentioned techniquesis actually a sum of thermal stress and intrinsic stress. Therefore, thenature of the residual stress is determined by the nature and relativemagnitude of these two stress components. Considering the effectof the thermal mismatch, we can assume the anatase thin film andsubstrate behave as a bi-metal plate, where the difference in thethermal expansion coefficients will induce stresses in both the filmand the substrate and will lead to the film/substrate bending uponcooling from the deposition temperature to room temperature. Inother words, the anatase film and substrate might be lattice matchedat the deposition temperature and are cooled to a lower temperature,so thermal strain is produced in the layer since the coefficients ofthermal expansion of the anatase film and substrate are not equal. As aresult, the anatase film is strained such that the in-plane latticeconstant of the anatase is the same as that of the substrate. The strainin this case is homogeneous and is known as the misfit strain. Beforethe deposition process, great care is taken to ensure the cleanliness ofthe substrates. Since the deposition temperature of the anatase isaround 300 °C, which is less than what is needed to grow thermaloxide (750 °C and 1100 °C), the anatase deposition on the Si substrateis assumed to be clean and free of the native amorphous SiO2 thinlayer. The resulting measurement stress of the anatase film on the Si(100) substrate gives more confidence assumptions.
4. Conclusions
X-ray diffraction confirmed the anatase TiO2 structure of the thinfilm and showed that the anatase film is crystalline material. The XRD
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data showed that the anatase film, 100 nm thick on the glass, hasa broader background that corresponds to the substrate, but thereis no evidence that this thinner film has an amorphous component.In contrast, Raman spectroscopy showed that this thin film is crystal-lite material. Raman spectroscopy measurements using the 514.5 nmexcitation wavelength of 144 cm−1 Eg vibration mode yielded infor-mation on the strain distribution as a function of film thickness.Curvature measurement and Raman spectroscopy were used forstress/strain analysis. The measurements of residual stressesfor crystalline anatase TiO2 thin film showed that all film are undera compressive stress. The stress/strain behavior for anatase film onthree different substrates showed that the strain for the thin filmon glass has a higher value compared to the strain on sapphire oron silicon substrates. The bi-axial strain originates from growth onlattice-mismatched substrates and from post-growth cooling. Thin100 nm anatase film on a glass substrate has the highest compres-sive stress of about 6000–7000MPa. Compressive stress in the anatasefilm on sapphire and silicon are considerably less (1200–2000 MPa).Both curvature measurements and Raman peak shift estimatedresidual stresses in anatase thin film compare well.
Acknowledgments
The authors wish to acknowledge the support of the Institute forManufacturing Research and the Smart Sensors and IntegratedMicrosystems (SSIM) program at Wayne State University.
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