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Effects of Absorption of Water and Other Reactive Species on the Fracture Properties of Organosilicate Thin Films Ting Y. Tsui a*, Joost J. Vlassak b, Kelly Taylor a, Andrew J. McKerrow a, and Robert Kraft a a Technology Development, Texas Instruments Incorporated, Dallas, TX b Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138-2901 * Contact author: [email protected]

Abstract This paper investigates the effects of the absorption of water and several other reactive species on the adhesion energy between organosilicate glass (OSG) thin films and various capping layers. The adhesion energy of these interfaces was measured by four-point bending tests conducted at room temperature in an ambient environment. The amount of absorbed reactant in the organosilicate glass films was controlled by exposing the capped organosilicate films to aqueous solutions for various periods of time. Results show that the initial degradation of the critical adhesion energy of the interfaces between OSG and , tantalum nitride (TaNx), (SiNx), and silicon dioxide (SiO2) capping layers is very fast, but that the adhesion energy reaches a saturation value thereafter. The reduced adhesion energy can be fully recovered to the “dry” value by baking the samples. A quantitative model based on Henry’s law is used to predict the adhesion degradation rate as a function of moisture exposure time. This model to a new and simple method for measuring the diffusion coefficient of water in OSG coatings or their interfaces.

Introduction Silica-based dielectrics are often doped with atoms and other organic molecules in order to reduce the permittivity of back-end-of-line (BEOL) inter-layer dielectrics. The resulting organosilicate glass (OSG) has a dielectric constant value less than 3.0 with approximately one half the of fused silica. The open network structure makes it easy for reactive molecules to diffuse and absorb into the OSG. Vlassak et al. [1-3] and Dauskardt et al. [4-6] have used the four-point-bend and double-cantilever beam techniques to demonstrate that the cohesive and adhesive fracture of OSG is strongly dependent on the chemical reactivity of the testing environment as measured by pH or relative humidity (% R.H.). However, there are very few, if any, studies to elucidate the effects of the absorption of water and other reactive species on the fracture behavior of OSG. The objective of this work is to quantitatively characterize this effect. The adhesion strength between OSG low-k films was measured as a function of exposure time to various aqueous solutions. The results demonstrate that the interfacial strength can be reduced by as much as 50% after a 2-week exposure. A diffusion model is used to extract the water diffusion coefficient in OSG from the time-dependent adhesion data.

Experimental Procedure Organosilicate glass thin films (OSG) with a thickness of 400 nm were deposited on 200 mm diameter bare silicon wafers at 400°C by using the plasma-enhanced chemical vapor deposition (PECVD) techniques. The precursor gases used in the deposition were 1, 3, 5, 7-tetra- methylcyclo-tetrasiloxane (TMCTS), , and . The OSG density measured by using X-Ray Reflectivity (XRR) is approximately 1.4 g/cm3; the relative dielectric constant of the OSG was 3.0. The precise chemical composition and mechanical properties of the OSG have been reported in previous studies by Vlassak and coworkers [1-3, 7]. Here, the effect of water diffusion on the adhesion between 400 nm thick OSG and four different capping materials is investigated – tantalum, tantalum nitride (TaN), silicon nitride (SiNx), and silicon dioxide (TEOS - SiO2). The last three capping layers are identical to those studied by Vlassak et al [1-3]. The TaNx coatings were deposited using reactive sputtering, followed by an in-situ sputter deposition of ~100 nm thick . The Ta coatings were sputter-deposited. The thickness of the barrier metal coatings is in the range of 10 to 20 nm. The 200 nm thick SiO2 and SiNx coatings were deposited at 400°C using PECVD. The adhesion energy of all samples was measured using the four-point bend technique [3, 8-9] conducted under ambient conditions with a temperature and relative humidity of 24 ± 2oC and 45 ± 5 %, respectively. Adhesion samples were prepared by cleaving 200 mm silicon substrates into 12 mm x 60 mm strips. These strips were then bonded face-to-face using and cured for one hour in an air convection oven at 120°C and a water partial pressure of (1.3 ± 0.1) x 103 Pa. After curing the epoxy, notches were made in the samples by using a saw. If P is the load at which the crack propagates along the interface, the corresponding energy release rate G is [3, 8-9] 21P2l2 ()1− ν 2 G = , (1) 16Eb2h3 where E and ν are the elastic modulus and Poisson’s ratio of the Si substrate, l the distance between the inner and outer loading pins in the four-point bending set-up, b the width of the specimen, and h the substrate thickness. The crack velocity was approximately 60 µm/s for all measurements. For each capping material, one group of samples was tested immediately after the epoxy was set. The rest of the specimens were submerged in de-ionized water at a temperature of 24 ± 2°C and tested after exposing them for specific periods of time (t). Results and Discussions The effect of water diffusion on fracture toughness Figure 1 shows the adhesion energy between the OSG and the various capping layers as a

7 12 200nm SiO /400nm OSG 1 day Ta/Cu 2 TaN/Cu 1600nm SiO /400nm OSG 10 6 2 SiN 1600nm SiO /3000nm OSG SiO 2 2 1 day 1 week 8 1 week 5 1 month

6 4 Baked SiN 4 3 Adhesion Energy, G (J/m^2) G Energy, Adhesion

Baked G (J/m^2) strength, Adhesion No water No exposure exposure SiO2 2 2 4 5 6 7 1000 104 105 106 107 1000 10 10 10 10 16hr 120C baked DI Water Soak Time (s) Moisture exposure time (s) Fig 1. Adhesion energy between OSG and various Fig 2. Adhesion energy of OSG/ SiO2 inter-faces with capping layers as a function of water exposure different film thickness values after soaking in time at 24oC. DI water.

2 function of submersion time in de-ionized water at 24°C. Each data point represents the average of at least five measurements and the error bars correspond to one standard deviation. X-ray photoelectron spectroscopy (XPS) of the fractures surfaces reveals that the SiNx, Ta, and TaNx- capped samples fractured cohesively. The adhesion energy of TaN/OSG interface is the largest. This can be explained by the pre-deposition plasma clean of the OSG surface, which removes surface contaminants and densifies the OSG surface. The crack path is located inside the OSG at a distance of approximately 5-10 nm from the interface. For the OSG/ SiO2 samples, by contrast, the XPS results show that the delamination occurred at the interface. This is consistent with observations reported by Vlassak et al. [3]. It is evident from Fig. 1 that the adhesion of each of the four film stacks degrades with increasing exposure time. The figure also reveals that the degradation of samples with SiO2 and SiNx caps can be recovered by baking the samples at 120oC for 16 hours. In order to eliminate the possibility that the time-dependent adhesion degradation is caused by changes in the mechanical properties of the epoxy as a result of the absorption of water, OSG/SiO2 samples with different bi-layer thicknesses were tested: 200 nm SiO2/420 nm OSG, 1600 nm SiO2/420 nm OSG, and 1600 nm SiO2/3000 nm OSG. If energy dissipation in the epoxy contributes significantly to the experimental adhesion values, the different film stacks should yield different adhesion values, because the stress distribution at the crack tip depends on the thicknesses of the films in the stack [10]. As illustrated in Fig. 2, this is clearly not the case: The initial adhesion values and the rates of degradation are indistinguishable within experimental scatter for the different film stacks. This indicates that the adhesion degradation is related to real changes that take place at the delaminating interface.

Model and discussion The solution of a one-dimensional diffusion problem that results in a water concentration profile in the OSG strip can be found in reference [11]. The adhesion strength (Ga) of the interface can be related to the moisture concentration by the following expression based on the assumption that water dissolves into the OSG film and follows Henry’s Law. G ()t = B − A()ln pv +Φ()γ ,τ , (2) a H 2O Dt where Φ()γ ,τ is a complicated function of the normalized time τ = and of the initial water a2 p content γ = i . A more detailed description of the diffusion model is given in reference [12]. pv H 2O The parameters A and B are material constants that describe the dependence of G at a given crack

Table 1. Fracture parameters for the OSG fracture calculations.

A 0.46

B 6.27

γ 0.0006

3

Fig. 3. Adhesion energy as a function of exposure time. The line represents the diffusion model. The arrows represent energy release rates for as-fabricated specimens tested in either dry N2 or at a relative humidity of 45%. velocity on the natural logarithm of the water partial pressure. This dependence is well established [13-14] and the values of these parameters were obtained from independent subcritical crack growth measurements for OSG coatings that are similar to the coatings used in this investigation [3]. The numerical values are listed in Table 1. The initial water content was determined from adhesion measurements of as-fabricated specimens in an N2 environment and is also given in Table 1. Equation (1) shows that the adhesion is a function of initial moisture v partial pressure (pi), saturated partial pressure (p H2O), moisture exposure time (t), sample width (a), and moisture diffusivity in OSG film. Figure 3 shows the experimental data for the OSG/SiO2 system along with the results obtained from Eq. (1), in which the diffusion coefficient was used as a fitting parameter. Agreement between the experimental data and the model calculations is very good and is shown in Fig. 3, The diffusion coefficient obtained from the model is D = 1.6 10-7 cm2/s.

Effects of other reactive species In the previous sections, we have demonstrated that the adhesion energy of the OSG interfaces is reduced by exposure to water in the environment before the adhesion . Recent work by Jacques et al. [15] and Vlassak et al. [3] shows that the fracture behavior of organosilicate glass (OSG) is also affected by the chemical reactivity of the environment as measured by pH. Similar results were also reported by Wiederhorn et al. [13-14] for bulk . However, the effect of the absorbed reactants into the OSG film or the interface on the cohesive and adhesion properties has not been characterized. In this part of the study, 420 nm thick OSG samples were capped with 1600 nm thick SiO2 films. The samples were submerged in a 3% solution (H2O2) or a 29% weight ammonium hydroxide solution (NH4OH) for various periods of time. The solutions have pH values of 5 + 1 and 13 + 1, respectively. The

4 5.5

3% H O 5 No moisture exposure 2 2 H O adhesion values 2 NH OH 4.5 4 1 week 4

3.5

3 120C / 24hrs

Adhesion G strength, (J/m^2) baked 2.5 1 day 2 4 5 6 7 1000 10 10 10 10 Reactant solution soak time (s) Fig 4. Adhesion strength of OSG/ SiO2 interfaces as a function of reactant solution exposure time. critical adhesion energy of the samples was measured by the four-point bend test in the ambient condition described above. For safety, samples were rinsed in DI water for 20 minutes after removing from the chemical solution. Figure 4 shows the OSG/SiO2 interfacial strength degrades with the exposure time in H2O2, NH4OH, and DI water. Interestingly, the results show that the rate of degradation is unchanged by the presence of H2O2 or NH4OH in the aqueous solution. This should be contrasted with results reported by Jacques et al. [15] and Vlassak et al. [3], who showed that the fracture energy of interfaces or the cohesive energy of -based materials degrades with the pH value of the environment during testing. A large effect of H2O2 has also been demonstrated [4]. XPS analysis shows that the failure occurs at the OSG/SiO2 interfaces regardless of the reactive species. Since the H2O2 or NH4OH in the solutions do not make the adhesion degradation worse - as one would expect based on the subcritical measurements in references [3] and [4] - the experimental data suggest that diffusion of these reactive species in OSG is much slower than that of water, possibly as a result of steric hinder. The adhesion degradation in this study is clearly caused by water. Figure 4 shows further that degradation can be recovered partially by a post-exposure bake at 120 °C for 24 hours. It is evident from the figure that the amount of recovery is the same for samples soaked in DI water or in NH4OH solutions. Both samples have a post-bake value of approximately 80% of the original “dry” value. This suggests that there is little or no permanent damage to the interface as a result of water absorption and that the degradation in the adhesion can indeed be attributed to the diffusion of water.

Conclusions 1. It has been demonstrated that the four-point-bend adhesion energy of the OSG films with dielectric and metal capping layers can be degraded by exposure to water in the environment prior to fracture. The amount of degradation can be as much as 50% after 2 weeks of exposure. The interfacial adhesion energy can be recovered by removing the absorbed water molecules using thermal treatments.

5 2. A quantitative diffusion model was used to describe the effect of water absorption on the adhesion degradation and to extract the diffusivity of water in the OSG coatings. 3. The amount of adhesion degradation as a result of exposure to H2O2 (pH~5) and NH4OH (pH~14) containing solutions is comparable to that for DI water.

References 1. Youbo Lin, Joost J. Vlassak, Ting Y. Tsui, Andrew J. McKerrow, Environmental Effects on Subcritical Delamination of Dielectric and Metal Films From Organosilicate Glass (OSG) Thin Films. MRS Proceedings, 2003. 766: p. E9.4. 2. Youbo Lin, Joost J. Vlassak, Ting Y. Tsui and Andrew J. McKerrow, Subcritical delamination of dielctric and metal films from low-k organosilicate glass (OSG) thin films in buffered pH solutions. Materials Research Symposium Proceedings, 2004. 795: p. 93-98. 3. Vlassak, J.J., Y. Lin, and T.Y. Tsui, Fracture of Organosilicate Glass Thin Films: Environmental Effects. and Engineering A, 2005. 391: p. 159-174. 4. Guyer, P.E. and R. Dauskardt, Fracture of nanoporous thin-film glasses. Materials, 2004. 3(1): p. 53-57. 5. Lane, M.W., J.M. Snodgrass, and R.H. Dauskardt, Environmental effects on interfacial adhesion. Microelectronics Reliability, 2001. 41: p. 1615-1624. 6. M. Lane, R. Dauskardt, Q. Ma, H. Fujimoto, and N. Krishna, Subcritical debonding of multilayer interconnect structures: temperature and humidity effects. Materials Research Society Proceedings, 1999. 563: p. 251. 7. Ting Y. Tsui, A. J. Griffin, Jr., Jeannette Jacques, Russell Fields, Andrew J. McKerrow, and Robert Kraft, Effects of Elastic Modulus on the Fracture Behavior of Low-Dielectric Constant Films Proceeding of the International Interconnect Technical Conference, IEEE, 2005. 8. Ma, Q., A four-point bending technique for studying subcritical crack growth in thin films and at interfaces. Journal of Materials Research, 1997. 12(3): p. 840-845. 9. Ting Tsui, Cindy Goldberg, Greg Braeckelman, Stan Filipiak, Bradley M. Ekstrom, J.J. Lee, Eric Jackson, Matthew Herrick, John Iacoponi, Jeremy Martin, and David Sieloff, The use of the four-point bending technique for determining the strength of low-k dielectric/barrier interface, Materials Research Society Proceedings, 2000. 612: p. D1.2.1 - D1.2.5 10. Suo, Z. and J.W. Hutchinson, Sandwich test specimens for measuring interface crack toughness. Materials Science and Engineering A, 1989. 107: p. 135-143. 11. Shewmon, P., Diffusion in . Second Edition ed. 1989, Warrendale, Pennsylvania: The , Metals, and Materials Society. 12. Ting Y. Tsui, Andrew J. McKerrow, and Joost J. Vlassak, Submitted to J. of Mechanics and Physics of Solids. 13. Wiederhorn, S.M., Influence of water vapor on crack propagation in soda- glass. Journal of the American Society, 1967. 50: p. 407-415. 14. Wiederhorn, S.M. and L.H. Bolz, Stress corrosion and static fatigue of glass. Journal of the American Ceramic Society, 1970. 53(10): p. 543-548. 15. Jeannette M. Jacques, Ting Y. Tsui, Andrew J. McKerrow, and Robert Kraft, Environmental Effects on Crack Characteristics for OSG Materials. Materials Research Society Proceedings, 2005. O10.6.

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