Fatigue in thin films Lifetime and damage formation.pdf

More documents

Recommendations

Info

922 O. Kraft et al. / Materials Science and Engineering A319–321 (2001) 919–923 3.2. Microbeam bending The grains in the Ag films on the SiO 2 microbeams were found to be columnar and had a median grain size of about 0.9 m independent of the film thickness. The majority of grains was (111)-oriented with a smaller fraction of (100)-oriented grains. The latter increased with increasing film thickness. As an example of a typical microbeam bending experiment, Fig. 4a shows the beam stiffness as function of the number of cycles for a beam with a 0.8 m thick film and a maximum strain max of 2.2%. Initially, the beam stiffness is nearly constant. The dramatic decrease in stiffness after approximately 6×10 −5 cycles indicates the onset of severe damage formation. Similar experiments conducted on samples with different film thicknesses are summarized in Fig. 4b. The filled symbols denote the experiments in which a distinct decrease in beam stiffness was observed within 3.9×10 6 cycles accompanied by crack formation. Only films with a thickness of 0.6 m or more suffered from fatigue damage. 3.3. Damage morphology Typical micrographs of fatigue damage are shown in Fig. 5. For both Cu and Ag films, the damage morphology can be described as follows, extrusions (marked as E) are formed within large grains, whereas intergranular cracks (C) were found in fine grained regions of the film. The extrusion height is comparable to the film thickness. Underneath the extrusions, large voids are found at the interface between the film and the substrate. Some more detailed observations of the fatigue damage in the Ag films are described in [12]. Fig. 4. (a) Stiffness of a microbeam with 0.8 m thick film cycled to max of 2.2% at a frequency of 45 Hz. The sudden decrease of stiffness indicates the onset of damage formation. (b) Map for damage formation as a function of Ag film thickness and maximum strain. Thick films showed fatigue damage within 3.9×10 6 cycles (filled symbols) whereas thin films did not show structural features of fatigue (open symbols). The transition occurs for 0.6 and 0.8 m thick films as function of maximum strain. increases, i.e. the sample stiffness decreases. After 10000 cycles, the strain range has reached the level of the bare substrate indicating that the film is no longer contributing to the sample stiffness. The number N f of cycles to failure in these experiments was defined as the interception of two linear curve fits as demonstrated in Fig. 3a. In Fig. 3b, this number is plotted versus the total strain range. As expected, the lifetime decreases with increasing strain range following a relationship as given by Eq. (1). 4. Discussion Two techniques have been developed to study the fatigue behavior of thin films on substrates. Central to both methods is the use of an elastic substrate, which acts as an antagonist to an external mechanical loading. As a result, films are deformed in tension on loading and in compression on unloading. Another important aspect of these experiments is that film fatigue damage is not catastrophic because the undamaged substrate maintains some mechanical integrity of the sample. However, the degradation of the film, i.e. crack formation and propagation, is manifested as a decrease in stiffness of the film–substrate composite. Based on this distinct decrease in stiffness, the lifetime of Cu films on polyimide substrates was determined as a function of applied total strain range. It was not possible to determine the plastic strain range, since the film stress was not measurable during the experiments. As a result, Fig. 3b is plotted as the initial total strain range as a
O. Kraft et al. / Materials Science and Engineering A319–321 (2001) 919–923 923 significantly slower or suppressed in thinner films, which would be qualitatively in agreement with the fact that the strength of metal films increases with decreasing film thickness [1,2,14]. However, it is currently not clear, if the formation of the extrusions is related to the occurrence of dislocation structures, such as persistent slip bands, as in bulk materials. More systematic studies, including TEM investigations, are currently pursued to obtain a deeper understanding of fatigue mechanisms in thin films. 5. Summary Fig. 5. Damage morphology after fatigue testing; (a) Focused ion beam micrograph of a 3 m thick Cu film on a polyimide substrate, large extrusions and cracks are marked by E and C, respectively. (b) SEM micrograph showing the 0.8 Ag film, from the test described in Fig. 4a, at the fixed end of a microbeam. Large extrusions (marked by E) are connected by a crack (C). function of N f . The data follow Eq. (1) with an exponent of −0.4, which is somewhat higher than typically observed values of −0.5 to −0.7 for many bulk metal materials [8]. This is partially accounted for by the use of the total strain range instead of the plastic strain range, because the difference between the two is more important at small strain ranges. As a result of this consideration, the exponent of −0.4 can be regarded as an upper bound. The microbeam bending experiments reveal that the fatigue behavior changes significantly, when the film thickness is reduced below 1 m. Fig. 4b shows that films thinner than 0.6 m did not fatigue within 3.9× 10 6 cycles. The formation of extrusions appears to be In an attempt to systematically study the fatigue behavior of thin metal films as a function of film thickness, grain size, and straining conditions, we have developed two new methods. Central to both methods is the use of an elastic substrate, which acts as an antagonist to an external mechanical loading. First results can be summarized as follows; fatigue damage includes the formation of large transgranular extrusions and intergranular cracks. The crack formation is associated with a distinct decrease in stiffness of the film– substrate composite. The lifetime of 3 m thick Cu films on polyimide substrates follows a Coffin– Manson-type relationship. No fatigue damage was found in Ag films with thicknesses below 0.6 m as tested by dynamic microbeam deflection. References [1] W.D. Nix, Met. Trans. A 20A (1989) 2217–2245. [2] R.-M. Keller, S.P. Baker, E. Arzt, J. Mater. Res. 13 (1998) 1307–1317. [3] E. Arzt, Acta Mater. 46 (1998) 5611–5626. [4] U. Essmann, H. Mughrabi, Phil. Mag. A 40 (1979) 731–756. [5] S. Hong, R. Weil, Thin Solid Films 283 (1996) 175–181. [6] M. Judelewicz, H.U. Künzi, N. Merk, B. Ilschner, Mat. Sci. Eng. A186 (1994) 135–142. [7] D.T. Read, Int. J. Fatigue 20 (1998) 203–209. [8] S. Suresh, Fatigue of Materials, Second ed, Cambridge University Press, Cambridge, 1999, pp. 137–139. [9] Y. Oshida, P.C. Chen, J. Electr. Packaging 113 (1991) 58–62. [10] H.D. Merchant, M.G. Minor, Y.L. Liu, J. Electron Mater. 28 (1999) 998–1007. [11] M. Hommel, O. Kraft, and E. Arzt. J. Mater. Res., 14 (1999). [12] R. Schwaiger, O. Kraft, Scr. Mat. 41 (1999) 823–829. [13] S.P. Baker, W.D. Nix, J. Mater. Res. 9 (1994) 3131–3144. [14] R. Venkatraman, J.C. Bravman, J. Mater. Res. 7 (1992) 2040.
Page 1 and 2: Materials Science and Engineering A
Page 3: O. Kraft et al. / Materials Science

Fatigue in thin films Lifetime and damage formation.pdf

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?