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Intrinsic, thermal and hygroscopic residual stresses in thin gas-barrier films on polymer substrates
Thin Solid Films 515 (2007) 7437 – 7441 www.elsevier.com/locate/tsf
Intrinsic, thermal and hygroscopic residual stresses in thin gas-barrier films on polymer substrates P. Dumont, G. Tornare, Y. Leterrier, J.-A.E. Månson ⁎ Laboratoire de Technologie des Composites et Polymères (LTC), Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 12, CH- 1015 Lausanne, Switzerland Available online 16 January 2007
Abstract Intrinsic, thermal, and hygroscopic contributions to the in-plane residual stress in silicon nitride films on polyimide substrates are identified, based on isohygric thermal ramps and isothermal relative humidity jumps, combined with non-linear elastic modeling of the resulting dynamics of film curvature. This approach enables the thermal and hygroscopic properties of thin nitride films to be determined and provides useful input for material and process control. © 2006 Elsevier B.V. All rights reserved. Keywords: SiNx; Polyimide; Residual stress; Gas-barrier
1. Introduction Flexible displays are complex multi-material structures in the form of layered stacks including a polymer or metal substrate, barrier and encapsulation layers, and thin film transistor devices. Residual stresses are inherent to this class of multilayer composite material, and accurate predictions of their dynamics are essential in optimizing the processing of the multilayer structures [1,2]. In the case of thin inorganic films deposited from a vapor phase onto polymer substrates, residual stresses comprise intrinsic, thermal, and hygroscopic contributions [3]. During deposition, intrinsic stresses arise from the out-ofequilibrium growth process of the film [4]. Thermal stresses build-up during cool-down from the deposition temperature due to the thermal mismatch between the different materials, and may further develop upon thermal cycling during service life. Similarly, hygroscopic stresses may result from the mismatch in water uptake behavior between the different materials. The present study focuses on silicon nitride films on polyimide substrates, developed to protect organic lightemitting devices from ingression of oxygen and moisture. The objective is to identify the intrinsic, thermal and hygroscopic contributions to the total residual stress. To this end, the curvature of the composite films subjected to iso-hygric thermal
⁎ Corresponding author. Tel.: +41 21 693 4281; fax: +41 21 693 5880. E-mail address: [email protected] (J.-A.E. Månson). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.104
ramps, and isothermal relative humidity jumps, was analyzed based on a non-linear elastic theoretical modeling framework. 2. Theory There exist numerous stress analyses relevant for bilayer (e.g., [3,5]) and multilayer structures [6]. Their validity depends on the thickness ratio between the coating layer and the substrate, and is limited to small strains. In the case of stiff coatings on polymer substrates, large out-of-plane deflections whose magnitude is comparable to or larger than the thickness of the composite film are frequently observed. To account for these, the present analysis uses von Karman's non-linear geometric strain–displacement relations [7–9] with the following assumptions: ▪ The film/substrate interface bond is perfect. ▪ The silicon nitride (SiNx) and polyimide (PI) materials are homogeneous, isotropic, and elastic (the viscoelastic behavior of the PI is considered to be negligible in the investigated temperature range). ▪ During deposition the PI substrate is maintained flat on a carrier system at a constant temperature of 200 °C and the coated PI is released from the carrier system at the end of the deposition. ▪ A uniform residual in-plane biaxial stress state is assumed in the thin film (thus all variations of the stress and strain fields near the edges of the plate structure are ignored).
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In this approach (case 3 in [7]), the in-plane coating stress depends on a bifurcation curvature: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðH4 −H1 Þ : ð1Þ jbif ¼ H3 Before this bifurcation, the curvatures κx = κy = κ and the stress writes as: rr;c s ¼
jðH3 j2 þ H1 þ H4 Þ : H2r
ð2Þ
After the bifurcation, the stress writes as: rr;c e ¼
H4 ðjx þ jy Þ ; H2r
jx jy ¼
H4 −H1 ¼ C ste ; H3
ð3Þ
where the parameters Hi(hs, hc, Lx, Ly, Es, νs, Ec, νc) depend on the geometrical parameters and on the elastic coefficients of both layers [7]. The temperature and moisture-dependent in-plane residual stress in the coating, σr,c, derived from the measured curvature of the composite film is then defined as the sum of a constant intrinsic stress, σint,c , a thermal stress, σth,c, and a hygroscopic stress, σhy,c: rr;c ¼ rint;c þ rth;c þ rhy;c Z Tc Ec int;c ¼r þ ½as ðT Þ−ac ðT ÞdT 1−mc T Z uc ref Ec þ ½b ðuÞ−bc ðuÞdu; 1−mc uref s
The Young's modulus of the SiNx layers was determined by means of nano-indentation tests as detailed elsewhere [10], and was found to be equal to 100 ± 10 GPa. Their Poisson coefficient was assumed to be equal to 0.26 [11]. The Young's modulus, Es, and Poisson coefficient, νs, of the polymer substrate were determined from tensile tests using dog-bone type samples of a reduced section equal to 6 × 50 mm2. Tests were carried out at a strain rate equal to 1.7 × 10− 3 s− 1 at six different constant temperatures (23, 60, 100, 150, 200 and 250 °C) by means of a UTS tensile test machine equipped with a furnace and a 1000 N load cell. The variations of Es in the range from 23 °C to 250 °C could be approximated by the linear fit Es = − 12.41 T + 5638.9, with Es in MPa and T in °C. The Poisson coefficient for the polyimide substrate was found to be equal to 0.25. The variations of the CTE αs of the polyimide substrate with respect to the temperature was determined by means of thermomechanical analysis, and was approximated by a second order polynomial in the range 25 ≤ T ≤ 250 °C: as ¼ 10−6 ð31:773−0:046674 T þ 0:00026144 T 2 Þ: The in-plane residual stress in the SiNx thin film was calculated from the main curvature κx of coated substrates placed in differently changing thermal or hygroscopic conditions. The curvature κx was calculated from the maximum deflection, δ, of the midplane of 55 × 6 mm2 rectangular samples freely supported by two razor blades with a separation, L, using a binocular lens (Olympus SZH): jx ¼
ð4Þ
where Ec and νc are the Young's modulus and Poisson coefficient of the coating, respectively. T is the temperature, Tc the actual temperature of the sample, Tref the deposition temperature, αs and αc the temperature-dependent coefficients of thermal expansion (CTE) of the PI substrate, and SiNx thin film, respectively. φ is the relative humidity, φc the actual relative humidity of the ambient air, φref the reference relative humidity chosen to be equal to 0, and βs and βc, are the coefficients of hygroscopic expansion (CHE) of the PI substrate and SiNx thin film, respectively. 3. Experimental section A 125 μm thick polyimide (PI) substrate (Upilex® S, UBE) was used. The PI foils were annealed at 120 °C during 30 min before deposition of SiNx coatings by plasma enhanced chemical vapor deposition (PECVD) at a temperature of 200 °C. Seven different coating thickness (50, 100, 200, 300, 400, 600 and 800 nm) were investigated.
8d : 4d2 þ L2
ð5Þ
Notice that the width of the sample was chosen such that it was wider than the extent of the damage zone induced in the SiNx thin film on the edges of the sample, which was found to be less than 100 μm wide. A minimum of two samples was tested for each SiNx thickness. Iso-hygric thermal ramps were carried out in a vacuum oven, i.e. at zero relative humidity. Samples were first heated under atmospheric pressure from room temperature to 170 °C, during which moisture present in the sample was evacuated. The sample was then cooled under vacuum (p b 50 mbar) down to room temperature at a slow rate equal to 0.8 K/min to ensure a homogeneous sample temperature. The deflection was measured throughout the temperature cycle, and its variation between 100 and 170 °C was extrapolated linearly to 200 °C (i.e. deposition temperature) in order to determine the intrinsic stress. Isothermal relative humidity (RH) jumps were carried out at 22 °C. Samples were placed in an environmental chamber equipped with an RH generator (VTI RH-200) controlled by a hygrometer (Ebro RHT 200). Samples were mounted in the chamber set at 50% RH and their deflection measured until moisture uptake equilibrium was reached (typically 2 h). The RH was then was set to 20%, 35%, and 80%, and the resulting change of deflection measured until a new equilibrium was
P. Dumont et al. / Thin Solid Films 515 (2007) 7437–7441
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Fig. 1. Thickness dependence of the total residual stress σr,c, intrinsic stress σint,c and thermal stress σth,c in SiNx coatings on a PI substrate at 22 °C and 0% RH.
Fig. 3. Influence of the SiNx coating thickness on the calculated CTE αc of the coating.
reached. It was observed that the deflection at equilibrium at a given RH was not influenced by the RH path (i.e., the deflection at 80% RH was the same after a jump from 20% RH or from 50% RH). In order to identify the origin of the intrinsic stress, the density of macro-defects [12] (also termed “pin-holes”) in the SiNx coating layer was calculated by using a reactive ion technique [13–15]. Atomic oxygen was used to erode the organic substrate at the location of defects in the inorganic coating, and the etched zones were subsequently detected under optical microscopy. Defect density was obtained from the
number of defects counted on 20 micrographs of area equal to 15,600 μm2.
Fig. 2. Influence of the SiNx coating thickness on macro-defect density (the solid line is a power-law fit of the data).
4. Intrinsic and thermal stresses Fig. 1 shows the intrinsic and thermal contributions to the total residual stress for the different coating thicknesses, at 22 °C and in the absence of moisture. Intrinsic stress (i.e., the stress at deposition temperature) was found to be constant within experimental scatter for thicknesses in the range from
Fig. 4. Thickness dependence of the total residual stress σr,c, intrinsic stress σint,c, thermal stress σth,c and hygroscopic stress σhy,c in SiNx coatings on a PI substrate at 22 °C and 50% RH.
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200 to 800 nm, where it is compressive and equal to approximately − 150 MPa. Moreover, it relaxed in the lowest thickness range considered, although experimental scatter was too large to draw further conclusions. As shown in Fig. 2, the density of macro-defects decreased for increasing thickness, and did not appear to correlate with the intrinsic stress data. The latter should in fact be related to the sub-micron disordered structure of the inorganic coating and related nano-defect population [12] that controls the transport of small molecules. Ongoing work is thus devoted to finding out whether the gas permeation properties of the SiNx/PI composite films have a bearing on the intrinsic stress. The thermal stress resulting from cooling from 200 °C to 22 °C was compressive and considerably higher than the intrinsic stress (Fig. 1). It was highest in absolute value in the low thickness range and relaxed to approximately − 500 MPa for thickness of 300 nm and above. This result was unexpected since the thermal load usually increases with deposition time, hence with increasing thickness. In the present case however, the substrate temperature was maintained constant during deposition. One may therefore invoke the lack of thermal stability of the polymer. This, however, was not the case, and it was verified that the curvature of the composite film was reversible upon thermal cycling up to 170 °C, so that the decrease of thermal stress with increasing coating thickness should be related to variations of the CTE of the nitride coating. As depicted in Fig. 3, the CTE of the coating lies in the range from 5 to 15 10− 6 K− 1 , with an average close to 10− 5 K− 1 that is higher than the value for bulk Si3N4 which is equal to 2.9·10 − 6 K− 1 [11]. With this, and the substrate CTE data, the thermal stress and total stress in the nitride coating on the PI substrate, and resulting curvature of the bilayer film, can be calculated for any moisture-free thermal path.
Fig. 5. Influence of the SiNx coating thickness on the CHE βc of the coating for four different values of the CHE βs of the polyimide substrate.
5. Hygroscopic stress The equilibrium hygroscopic stress at 22 °C and 50% RH for three different coating thickness is shown in Fig. 4. In contrast to the other stress contributions also shown in the figure, the hygroscopic stress is tensile, with an average of 430 MPa, comparable in magnitude to the thermal stress. This data was used to evaluate the CHE of the nitride coating using the derivative of Eq. (4) with respect to the relative humidity. To this end, different values of the unknown CHE βs of the substrate were used [16] and the results are reported in Fig. 5. The large experimental scatter did not enable accurate values to be determined, and a zero CHE for the nitride is consistent with the considered CHE values of polyimide [3,16]. Thus the hygroscopic stress should be mainly related to the polyimide expansion upon moisture uptake. Providing that the latter polymer CHE is available, the above analysis can be used to calculate the residual stress and resulting curvature of the bilayer film for any hygro-thermal path. 6. Conclusions The intrinsic, thermal, and hygroscopic contributions to the residual stress in silicon nitride films on polyimide substrates were investigated. Curvature changes of composite samples subjected to specific hygro-thermal paths were analyzed using a non-linear elastic framework. Intrinsic stress was found to be compressive and equal to − 150 MPa. It was independent of coating thickness, with no correlation to the macro-defect density. Thermal stress was also compressive and equal to approximately − 500 MPa at 22 °C. The CTE of the nitride coatings was found to vary with thickness, with an average value equal to approximately 10− 5 K− 1. Hygroscopic stress was tensile, and, at 50% RH, was comparable in magnitude to the thermal stress. The analysis of the results was consistent with a zero CHE for the silicon nitride in spite of a large experimental scatter. The total residual stress at 22 °C and 50% RH was found to be compressive and to increase in absolute value with coating thickness, from − 100 MPa at 200 nm to − 500 MPa at 800 nm. List of symbols αs, αc Coefficients of thermal expansion (CTE) of substrate and coating βs, βc Coefficients of hygroscopic expansion (CHE) of substrate and coating δ Maximum deflection during curvature measurement Ec, νc Coating Young's modulus and Poisson coefficient Es, νs Substrate Young's modulus and Poisson coefficient φ, φc, φref Relative humidity, actual relative humidity of ambient air, reference relative humidity κ, κbif, κx, κy Curvature, and curvatures at bifurcation and along x and y directions hc, hs Coating and substrate thicknesses H1, H3, H4, H2σ Elastic functions L Support separation during curvature measurement Lx, Ly Sample in-plane dimensions along x and y axis
P. Dumont et al. / Thin Solid Films 515 (2007) 7437–7441
σr,c, σsr,c, σer,c Coating residual stress, and values before and after bifurcation σint,c , σth,c, σhy,c Intrinsic, thermal and hygroscopic contributions to coating stress T, Tc, Tref Temperature, actual temperature, deposition temperature Acknowledgments The authors are grateful to the EU-funded Flexidis project (IST- 004354) for funding this work, to Unaxis France, Display Technology for the supply of film samples, and to the EPFL Center for MicroNanoTechnology (CMI) for the reactive ion etching facility. References [1] Y. Leterrier, Y. Wyser, J.-A.E. Månson, J. Adhes. Sci. Technol. 15 (2001) 841. [2] G. Rochat, Y. Leterrier, P. Fayet, J.-A.E. Månson, Thin Solid Films 484/1-2 (2005) 94. [3] S. Wagner, H. Gleskova, I.-C. Cheng, J.C. Sturm, Z. Suo, Flexible Flat Panel Displays, Wiley, New York, 2005.
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[4] F.M. d'Heurle, J.M.E. Harper, Thin Solid Films 171 (1989) 81. [5] M. Benabdi, A.A. Roche, J. Adhes. Sci. Technol. 11 (1997) 281. [6] P.H. Townsend, D.M. Barnett, T.A. Brunner, J. Appl. Phys. 62 (1987) 4438. [7] C.B. Masters, N.J. Salamon, Int. J. Eng. Sci. 31 (1993) 915. [8] J. Gunnars, U. Wiklund, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 336 (2002) 7. [9] D.E. Fahnline, C.B. Masters, N.J. Salamon, J. Vac. Sci. Technol., A, Vac. Surf. Films 9 (1991) 2483. [10] A.V. Dijken, J.D. Toonder, P. Bouten, Nano Indentations on Inorganic Coatings for the FlexiDis Project, Technical Report, Philips Research Laboratories, Eindhoven, The Netherlands, 2005. [11] M. Jiang, N.O. Wood, R. Komanduri, Wear 220 (1998) 59. [12] A.P. Roberts, B.M. Henry, A.P. Sutton, C.R.M. Grovenor, G.A.D. Briggs, T. Miyamoto, A. Kano, Y. Tsukahara, M. Yanaka, J. Membr. Sci. 208 (2002) 75. [13] A.S. Da Silva Sobrinho, G. Czeremuszkin, M. Latrèche, M.R. Wertheimer, J. Vac. Sci. Technol., A, Vac. Surf. Films 18 (2000) 149. [14] G. Rochat, Y. Leterrier, L. Garamszegi, J.-A.E. Månson, P. Fayet, Surf. Coat. Technol. 174–175 (2003) 1029. [15] G. Rochat, Y. Leterrier, P. Fayet, J.-A.E. Månson, Surf. Coat. Technol. 200 (2005) 2236. [16] S. Murray, G. Hillman, M. Pecht, J. Electron. Packag. 126 (2004) 390.
Intrinsic, thermal and hygroscopic residual stresses in thin gas-barrier films on polymer substrates P. Dumont, G. Tornare, Y. Leterrier, J.-A.E. Månson ⁎ Laboratoire de Technologie des Composites et Polymères (LTC), Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 12, CH- 1015 Lausanne, Switzerland Available online 16 January 2007
Abstract Intrinsic, thermal, and hygroscopic contributions to the in-plane residual stress in silicon nitride films on polyimide substrates are identified, based on isohygric thermal ramps and isothermal relative humidity jumps, combined with non-linear elastic modeling of the resulting dynamics of film curvature. This approach enables the thermal and hygroscopic properties of thin nitride films to be determined and provides useful input for material and process control. © 2006 Elsevier B.V. All rights reserved. Keywords: SiNx; Polyimide; Residual stress; Gas-barrier
1. Introduction Flexible displays are complex multi-material structures in the form of layered stacks including a polymer or metal substrate, barrier and encapsulation layers, and thin film transistor devices. Residual stresses are inherent to this class of multilayer composite material, and accurate predictions of their dynamics are essential in optimizing the processing of the multilayer structures [1,2]. In the case of thin inorganic films deposited from a vapor phase onto polymer substrates, residual stresses comprise intrinsic, thermal, and hygroscopic contributions [3]. During deposition, intrinsic stresses arise from the out-ofequilibrium growth process of the film [4]. Thermal stresses build-up during cool-down from the deposition temperature due to the thermal mismatch between the different materials, and may further develop upon thermal cycling during service life. Similarly, hygroscopic stresses may result from the mismatch in water uptake behavior between the different materials. The present study focuses on silicon nitride films on polyimide substrates, developed to protect organic lightemitting devices from ingression of oxygen and moisture. The objective is to identify the intrinsic, thermal and hygroscopic contributions to the total residual stress. To this end, the curvature of the composite films subjected to iso-hygric thermal
⁎ Corresponding author. Tel.: +41 21 693 4281; fax: +41 21 693 5880. E-mail address: [email protected] (J.-A.E. Månson). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.104
ramps, and isothermal relative humidity jumps, was analyzed based on a non-linear elastic theoretical modeling framework. 2. Theory There exist numerous stress analyses relevant for bilayer (e.g., [3,5]) and multilayer structures [6]. Their validity depends on the thickness ratio between the coating layer and the substrate, and is limited to small strains. In the case of stiff coatings on polymer substrates, large out-of-plane deflections whose magnitude is comparable to or larger than the thickness of the composite film are frequently observed. To account for these, the present analysis uses von Karman's non-linear geometric strain–displacement relations [7–9] with the following assumptions: ▪ The film/substrate interface bond is perfect. ▪ The silicon nitride (SiNx) and polyimide (PI) materials are homogeneous, isotropic, and elastic (the viscoelastic behavior of the PI is considered to be negligible in the investigated temperature range). ▪ During deposition the PI substrate is maintained flat on a carrier system at a constant temperature of 200 °C and the coated PI is released from the carrier system at the end of the deposition. ▪ A uniform residual in-plane biaxial stress state is assumed in the thin film (thus all variations of the stress and strain fields near the edges of the plate structure are ignored).
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In this approach (case 3 in [7]), the in-plane coating stress depends on a bifurcation curvature: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðH4 −H1 Þ : ð1Þ jbif ¼ H3 Before this bifurcation, the curvatures κx = κy = κ and the stress writes as: rr;c s ¼
jðH3 j2 þ H1 þ H4 Þ : H2r
ð2Þ
After the bifurcation, the stress writes as: rr;c e ¼
H4 ðjx þ jy Þ ; H2r
jx jy ¼
H4 −H1 ¼ C ste ; H3
ð3Þ
where the parameters Hi(hs, hc, Lx, Ly, Es, νs, Ec, νc) depend on the geometrical parameters and on the elastic coefficients of both layers [7]. The temperature and moisture-dependent in-plane residual stress in the coating, σr,c, derived from the measured curvature of the composite film is then defined as the sum of a constant intrinsic stress, σint,c , a thermal stress, σth,c, and a hygroscopic stress, σhy,c: rr;c ¼ rint;c þ rth;c þ rhy;c Z Tc Ec int;c ¼r þ ½as ðT Þ−ac ðT ÞdT 1−mc T Z uc ref Ec þ ½b ðuÞ−bc ðuÞdu; 1−mc uref s
The Young's modulus of the SiNx layers was determined by means of nano-indentation tests as detailed elsewhere [10], and was found to be equal to 100 ± 10 GPa. Their Poisson coefficient was assumed to be equal to 0.26 [11]. The Young's modulus, Es, and Poisson coefficient, νs, of the polymer substrate were determined from tensile tests using dog-bone type samples of a reduced section equal to 6 × 50 mm2. Tests were carried out at a strain rate equal to 1.7 × 10− 3 s− 1 at six different constant temperatures (23, 60, 100, 150, 200 and 250 °C) by means of a UTS tensile test machine equipped with a furnace and a 1000 N load cell. The variations of Es in the range from 23 °C to 250 °C could be approximated by the linear fit Es = − 12.41 T + 5638.9, with Es in MPa and T in °C. The Poisson coefficient for the polyimide substrate was found to be equal to 0.25. The variations of the CTE αs of the polyimide substrate with respect to the temperature was determined by means of thermomechanical analysis, and was approximated by a second order polynomial in the range 25 ≤ T ≤ 250 °C: as ¼ 10−6 ð31:773−0:046674 T þ 0:00026144 T 2 Þ: The in-plane residual stress in the SiNx thin film was calculated from the main curvature κx of coated substrates placed in differently changing thermal or hygroscopic conditions. The curvature κx was calculated from the maximum deflection, δ, of the midplane of 55 × 6 mm2 rectangular samples freely supported by two razor blades with a separation, L, using a binocular lens (Olympus SZH): jx ¼
ð4Þ
where Ec and νc are the Young's modulus and Poisson coefficient of the coating, respectively. T is the temperature, Tc the actual temperature of the sample, Tref the deposition temperature, αs and αc the temperature-dependent coefficients of thermal expansion (CTE) of the PI substrate, and SiNx thin film, respectively. φ is the relative humidity, φc the actual relative humidity of the ambient air, φref the reference relative humidity chosen to be equal to 0, and βs and βc, are the coefficients of hygroscopic expansion (CHE) of the PI substrate and SiNx thin film, respectively. 3. Experimental section A 125 μm thick polyimide (PI) substrate (Upilex® S, UBE) was used. The PI foils were annealed at 120 °C during 30 min before deposition of SiNx coatings by plasma enhanced chemical vapor deposition (PECVD) at a temperature of 200 °C. Seven different coating thickness (50, 100, 200, 300, 400, 600 and 800 nm) were investigated.
8d : 4d2 þ L2
ð5Þ
Notice that the width of the sample was chosen such that it was wider than the extent of the damage zone induced in the SiNx thin film on the edges of the sample, which was found to be less than 100 μm wide. A minimum of two samples was tested for each SiNx thickness. Iso-hygric thermal ramps were carried out in a vacuum oven, i.e. at zero relative humidity. Samples were first heated under atmospheric pressure from room temperature to 170 °C, during which moisture present in the sample was evacuated. The sample was then cooled under vacuum (p b 50 mbar) down to room temperature at a slow rate equal to 0.8 K/min to ensure a homogeneous sample temperature. The deflection was measured throughout the temperature cycle, and its variation between 100 and 170 °C was extrapolated linearly to 200 °C (i.e. deposition temperature) in order to determine the intrinsic stress. Isothermal relative humidity (RH) jumps were carried out at 22 °C. Samples were placed in an environmental chamber equipped with an RH generator (VTI RH-200) controlled by a hygrometer (Ebro RHT 200). Samples were mounted in the chamber set at 50% RH and their deflection measured until moisture uptake equilibrium was reached (typically 2 h). The RH was then was set to 20%, 35%, and 80%, and the resulting change of deflection measured until a new equilibrium was
P. Dumont et al. / Thin Solid Films 515 (2007) 7437–7441
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Fig. 1. Thickness dependence of the total residual stress σr,c, intrinsic stress σint,c and thermal stress σth,c in SiNx coatings on a PI substrate at 22 °C and 0% RH.
Fig. 3. Influence of the SiNx coating thickness on the calculated CTE αc of the coating.
reached. It was observed that the deflection at equilibrium at a given RH was not influenced by the RH path (i.e., the deflection at 80% RH was the same after a jump from 20% RH or from 50% RH). In order to identify the origin of the intrinsic stress, the density of macro-defects [12] (also termed “pin-holes”) in the SiNx coating layer was calculated by using a reactive ion technique [13–15]. Atomic oxygen was used to erode the organic substrate at the location of defects in the inorganic coating, and the etched zones were subsequently detected under optical microscopy. Defect density was obtained from the
number of defects counted on 20 micrographs of area equal to 15,600 μm2.
Fig. 2. Influence of the SiNx coating thickness on macro-defect density (the solid line is a power-law fit of the data).
4. Intrinsic and thermal stresses Fig. 1 shows the intrinsic and thermal contributions to the total residual stress for the different coating thicknesses, at 22 °C and in the absence of moisture. Intrinsic stress (i.e., the stress at deposition temperature) was found to be constant within experimental scatter for thicknesses in the range from
Fig. 4. Thickness dependence of the total residual stress σr,c, intrinsic stress σint,c, thermal stress σth,c and hygroscopic stress σhy,c in SiNx coatings on a PI substrate at 22 °C and 50% RH.
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200 to 800 nm, where it is compressive and equal to approximately − 150 MPa. Moreover, it relaxed in the lowest thickness range considered, although experimental scatter was too large to draw further conclusions. As shown in Fig. 2, the density of macro-defects decreased for increasing thickness, and did not appear to correlate with the intrinsic stress data. The latter should in fact be related to the sub-micron disordered structure of the inorganic coating and related nano-defect population [12] that controls the transport of small molecules. Ongoing work is thus devoted to finding out whether the gas permeation properties of the SiNx/PI composite films have a bearing on the intrinsic stress. The thermal stress resulting from cooling from 200 °C to 22 °C was compressive and considerably higher than the intrinsic stress (Fig. 1). It was highest in absolute value in the low thickness range and relaxed to approximately − 500 MPa for thickness of 300 nm and above. This result was unexpected since the thermal load usually increases with deposition time, hence with increasing thickness. In the present case however, the substrate temperature was maintained constant during deposition. One may therefore invoke the lack of thermal stability of the polymer. This, however, was not the case, and it was verified that the curvature of the composite film was reversible upon thermal cycling up to 170 °C, so that the decrease of thermal stress with increasing coating thickness should be related to variations of the CTE of the nitride coating. As depicted in Fig. 3, the CTE of the coating lies in the range from 5 to 15 10− 6 K− 1 , with an average close to 10− 5 K− 1 that is higher than the value for bulk Si3N4 which is equal to 2.9·10 − 6 K− 1 [11]. With this, and the substrate CTE data, the thermal stress and total stress in the nitride coating on the PI substrate, and resulting curvature of the bilayer film, can be calculated for any moisture-free thermal path.
Fig. 5. Influence of the SiNx coating thickness on the CHE βc of the coating for four different values of the CHE βs of the polyimide substrate.
5. Hygroscopic stress The equilibrium hygroscopic stress at 22 °C and 50% RH for three different coating thickness is shown in Fig. 4. In contrast to the other stress contributions also shown in the figure, the hygroscopic stress is tensile, with an average of 430 MPa, comparable in magnitude to the thermal stress. This data was used to evaluate the CHE of the nitride coating using the derivative of Eq. (4) with respect to the relative humidity. To this end, different values of the unknown CHE βs of the substrate were used [16] and the results are reported in Fig. 5. The large experimental scatter did not enable accurate values to be determined, and a zero CHE for the nitride is consistent with the considered CHE values of polyimide [3,16]. Thus the hygroscopic stress should be mainly related to the polyimide expansion upon moisture uptake. Providing that the latter polymer CHE is available, the above analysis can be used to calculate the residual stress and resulting curvature of the bilayer film for any hygro-thermal path. 6. Conclusions The intrinsic, thermal, and hygroscopic contributions to the residual stress in silicon nitride films on polyimide substrates were investigated. Curvature changes of composite samples subjected to specific hygro-thermal paths were analyzed using a non-linear elastic framework. Intrinsic stress was found to be compressive and equal to − 150 MPa. It was independent of coating thickness, with no correlation to the macro-defect density. Thermal stress was also compressive and equal to approximately − 500 MPa at 22 °C. The CTE of the nitride coatings was found to vary with thickness, with an average value equal to approximately 10− 5 K− 1. Hygroscopic stress was tensile, and, at 50% RH, was comparable in magnitude to the thermal stress. The analysis of the results was consistent with a zero CHE for the silicon nitride in spite of a large experimental scatter. The total residual stress at 22 °C and 50% RH was found to be compressive and to increase in absolute value with coating thickness, from − 100 MPa at 200 nm to − 500 MPa at 800 nm. List of symbols αs, αc Coefficients of thermal expansion (CTE) of substrate and coating βs, βc Coefficients of hygroscopic expansion (CHE) of substrate and coating δ Maximum deflection during curvature measurement Ec, νc Coating Young's modulus and Poisson coefficient Es, νs Substrate Young's modulus and Poisson coefficient φ, φc, φref Relative humidity, actual relative humidity of ambient air, reference relative humidity κ, κbif, κx, κy Curvature, and curvatures at bifurcation and along x and y directions hc, hs Coating and substrate thicknesses H1, H3, H4, H2σ Elastic functions L Support separation during curvature measurement Lx, Ly Sample in-plane dimensions along x and y axis
P. Dumont et al. / Thin Solid Films 515 (2007) 7437–7441
σr,c, σsr,c, σer,c Coating residual stress, and values before and after bifurcation σint,c , σth,c, σhy,c Intrinsic, thermal and hygroscopic contributions to coating stress T, Tc, Tref Temperature, actual temperature, deposition temperature Acknowledgments The authors are grateful to the EU-funded Flexidis project (IST- 004354) for funding this work, to Unaxis France, Display Technology for the supply of film samples, and to the EPFL Center for MicroNanoTechnology (CMI) for the reactive ion etching facility. References [1] Y. Leterrier, Y. Wyser, J.-A.E. Månson, J. Adhes. Sci. Technol. 15 (2001) 841. [2] G. Rochat, Y. Leterrier, P. Fayet, J.-A.E. Månson, Thin Solid Films 484/1-2 (2005) 94. [3] S. Wagner, H. Gleskova, I.-C. Cheng, J.C. Sturm, Z. Suo, Flexible Flat Panel Displays, Wiley, New York, 2005.
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