Very high cycle fatigue crack initiation in electroplated Ni films under extreme stress gradients

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Scripta Materialia 67 (2012) 45–48 www.elsevier.com/locate/scriptamat

Very high cycle fatigue crack initiation in electroplated Ni films under extreme stress gradients E.K. Baumert and O.N. Pierron⇑ G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, USA Received 6 February 2012; accepted 15 March 2012 Available online 20 March 2012

A characterization technique based on kilohertz micro-resonators is presented to investigate the very high cycle fatigue behavior of 20 lm thick electroplated Ni films with a columnar microstructure (grain diameter less than 2 lm). The films exhibit superior fatigue resistance due to the extreme stress gradients at the surface. The effects of stress amplitude and environment on the formation of fatigue extrusions and micro-cracks are discussed based on scanning electron microscopy and the tracking of the specimens’ resonant frequency. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thin films; Electroplating; Nickel; Fatigue

Electroplated Ni films are used as structural materials for a variety of micro-electro-mechanical system (MEMS) devices, including RF devices, relays [1], and resonant micro-machines [2] (e.g. accelerometers and angular rate sensors). As such, several fatigue reliability studies of electroplated Ni films were carried out over the last decade [3–6]. These experimental studies consisted of micro-scale versions of conventional bulk fatigue tests, under tension–tension [4–6] or fully reversed bending [3] loading at frequencies ranging from 10 to 200 Hz. These studies pointed to an apparent fatigue limit (at 107 cycles) roughly equal to 30 ± 10% of the ultimate tensile strength, commensurate with bulk Ni fatigue properties. However, the conventional fatigue “endurance” does not capture the actual fatigue behavior in the very high cycle fatigue (VHCF) regime (fatigue life, Nf > 108 cycles), which is more representative of resonant-type MEMS devices [2]. Accordingly, this letter introduces a fatigue characterization MEMS resonator operating at 8 kHz to study the VHCF properties of electrodeposited Ni thin films. In addition to the aforementioned engineering considerations pertaining to MEMS reliability studies, fatigue studies on thin film materials are motivated by the scientific curiosity to understand size effects governing fatigue mechanisms. For example, Kraft and coworkers [7] studied the effect of Cu film thickness and grain size

⇑ Corresponding author. Tel.: +1 404 894 7877; e-mail: olivier.pierron @me.gatech.edu

on the dislocation arrangements and resulting surface fatigue cracks in the low cycle fatigue (LCF) regime. They observed a transition at 1 lm (for either film thickness or grain size) from “bulk-like” behavior (formation of persistent slip bands (PSBs) and extrusions) to “small volume” behavior (individual dislocations and interfacemediated damage). Also, Boyce et al. [3,6] observed a surprising effect of air on the room temperature fatigue behavior of electrodeposited Ni films in the high cycle fatigue (HCF) regime, namely the formation of very thick oxides (up to 400 nm) at the surface of grains with extrusions. Comparatively, very few studies have focused on the governing fatigue mechanisms for metallic thin films in the VHCF regime. It is now well established that extrusions and intrusions (stage I micro-cracks) can develop in bulk ductile single-phase metals for plastic strain amplitudes below the PSB threshold after a large number of cycles (>108–109) [8,9]. In that regime, it is believed that gradual, slow roughening occurs due to irreversible (albeit small) cyclic plastic deformation, resulting in PSB formation due to local surface stress raisers [10]. The focus of this letter is to present a study on the VHCF crack initiation process in electroplated Ni films. In addition to the small dimensions and grain sizes that differentiate them from bulk materials, thin films may also experience large stress gradients in actual MEMS configurations [3,11], which cannot be studied with the conventional dogbone-shaped micro-tensile specimens. In contrast, the experimental setup presented next includes the effect of large stress gradients.

1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.03.017

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elastic modulus of Ni, ENi = 172 GPa. This ENi value is 7% higher than the one calculated in Ref. [1] for nominally identical films (160 GPa), but matches theoretical E values for h0 0 1i-textured Ni films (170– 177 GPa [4]) very well. Using ENi = 172 GPa, an elastic finite element method was used to calculate the maximum stress at the notch, rmax, as a function of angular rotation,2 h: rmax ðMPaÞ ¼ 62:6h ðmradÞ

ð1Þ

as well as the normalized stress gradient, g, over the first 2 lm from the notch:   1 dr  0:36 lm1 ð2Þ g¼ rmax dx 06x62lm

Figure 1. SEM images of the micro-resonator: (a) top-down view; (b) inclined view of the notched beam; (c) SEM image of the sidewall showing columnar microstructure (after ion beam etching).

The fatigue specimens shown in Figure 1 were fabricated with the commercially available MetalMUMPs process, in which the structural layer is electroplated Ni (20 lm thick), with a bottom 0.5 lm thick plating Cu base layer, and a top 0.5 lm thick Au layer [1]. The films exhibit a columnar microstructure (see Fig. 1c), with columnar grains typically 5–10 lm in height, and 1–2 lm in width, and small (<1 lm diameter) equiaxed grains in between the larger columnar grains. The measured yield stress, r0.2%, is 650 MPa and the ultimate tensile stress, ruts, is 870 MPa,1 values consistent with other studies given the smaller average grain size in this study [3–6]. Although not measured in this study, these films are expected to have a strong h0 0 1i out-of-plane texture [3–6]. The fatigue specimens are in-plane micro-resonators consisting of a notched cantilever beam (38.3 lm long, 14.8 lm wide, with a 3.9 lm root-radius notch 9.9 lm from the base; see Fig. 1a and b) and a fan-shaped mass with two “comb” structures on each side, for electrostatic actuation and capacitive sensing of motion [12]. The specimens are driven at resonance (8 kHz) throughout the test, leading to fully reversed loading conditions at the notch, and the resonant frequency, f0, is periodically measured using “frequency sweeps” [12]. The tests are performed in an environmental chamber (ESPEC SH-241). A three-dimensional finite element model of the entire structure was used to calculate f0 (modal analysis) and to calculate the stress distribution at the notch (structural elastic analysis). Including the effects of the bottom Cu and top Au layers (assuming bulk properties for these layers), the calculated f0 matched the measured value (7890 Hz) for an 1

These values are the average of five micro-tensile tests at room temperature, under a strain rate of 2  104 s1.

Bulk ultrasonic fatigue tests typically require external cooling to minimize temperature (T) increases due to dislocation damping [8,9]. Here, the first order T coefficient of frequency, Tf0 = 1/f0  (df0/dT), was measured between 30 and 80 °C for stress amplitudes, ra (ra = rmax), ranging from 200 to 425 MPa, and was independent of ra. The average value, 210 ppm °C–1, is close to the approximated predicted value, Tf0 = 1/2TE1/ 2Tq = 236 ppm °C1,3 meaning that no significant increase in T occurs at the notch even when ra approaches the yield stress. This result appears reasonable given that very small volumes experience plastic deformation and that these films exhibit large surface-to-volume ratios. Fifteen tests were performed at 30 °C, 50% RH and six tests were performed at 80 °C, 90% RH for ra ranging from 100 to 520 MPa (60% of ruts). The tests are considered stress-controlled, given that the output voltage (proportional to h) is typically constant during a test. None of the tested specimens failed after numbers of cycles ranging from 1 to 4  109 cycles, in sharp contrast to previous fatigue studies on electroplated Ni films (Nf ranging from 104 to 106–107 for ra between 200 and 300 MPa, with a fatigue “limit” at 20–45% of ruts [3–6]). This result highlights the strong dependence of the fatigue behavior of thin films on stress gradients, which can be orders of magnitude larger than in bulk materials.4 In this study, the stress gradient is characterized by Eq. (2) for the first 2 lm, meaning the average stress over this distance is 36% lower than the maximum value, ra. While also very high, the normalized stress gradient in [3], g = 0.08 lm1 did not appear to significantly influence the overall stress-life fatigue curve com2

The angular rotation h is measured at the end of each test, using a high magnification optical system, with a resolution of 0.3 mrad, from which the stress amplitude is calculated using this equation. At 80 °C, ENi is assumed to be 166.5 GPa (dE/dT = 0.11 GPa °C1[4]). 3 Using TE = 1/E  dE/dT = 525 ppm °C–1 [4] and Tq = 1/q  dq/ dT = 52 ppm °C–1. 4 Another effect may be the strain rate effect, which could lead to higher r0.2% and ruts values during the fatigue tests (e  50–100 s1 compared to 2  104 s1 during the microtensile measurements). However, large strain rate exponent values (m ¼ @ ln r=@ ln e  0:01  0:02) were only observed for face-centered cubic (fcc) metals with much smaller average grain sizes (<300–400 nm) [13]. Hence, the conventional m value for fcc metals, m  0.004, may be more appropriate for these films, leading to a small (5%) increase in r0.2% and ruts values.

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0 −10 −20 −30

Δ

−40 −50 30 ºC, 50% RH 80 ºC, 90% RH

−60

315 MPa 465 MPa 475 MPa

−70 −80

0

0.5

1

1.5

2 N (−)

2.5

285 MPa 400 MPa 425 MPa 3

3.5

4 x 109

Figure 3. Characteristic f0 evolution plots for films tested at 30 °C, 50% RH and at 80 °C 90% RH. See SEM images in Figure 2 for the following tests: ra = 315 MPa at 30 °C, 50% RH, ra = 465 MPa at 30 °C, 50% RH, and ra = 425 MPa at 80 °C, 90% RH. Arrows indicate discontinuities (decreases) in f0.

Figure 2. SEM images of the Ni films’ sidewalls after fatigue testing. (a) Low magnification image showing localized fatigue damage at notch root. High magnification images at notch root after: (b) ra = 315 MPa, N = 1.6  109 cycles at 30 °C, 50% RH; (c) ra = 425 MPa, N = 2.8  109 cycles at 30 °C, 50% RH; (d) ra = 470 MPa, N = 3.6  109 cycles at 30 °C, 50% RH; (e) ra = 425 MPa, N = 1.8  109 cycles at 80 °C, 90% RH. Arrows indicate micro-cracks.

pared to other studies under uniform stress states [4–6]. This result warrants further studies on the specific influence of extreme stress gradients on the fatigue resistance of electroplated Ni thin films. Examination of the sidewall surface at the notch root was performed for each specimen after completion or interruption of the fatigue tests, using scanning electron microscopy (SEM). At 30 °C, 50% RH, no surface degradation was observed for ra below 250 MPa. At 300– 325 MPa, a few small extrusions (less than 100 nm in height) were observed at the notch root (see Fig. 2b). Above 350 MPa, more extrusions were observed at the notch root, along with some intrusions (micro-cracks) at the extrusions’ edges (see Fig. 2c and d). The extrusions are typically less than 1–2 lm wide, and less than 300 nm in height. A test interrupted after 9  108 cycles and stopped after 2  109 cycles revealed more extrusions during the second examination, suggesting that these extrusions form in the VHCF regime. In addition to SEM examination, the f0 evolution was monitored during each test. Figure 3 shows representative f0 evolution plots for six tests. At 30 °C 50% RH, f0 typically decreases throughout the test, with much reduced (although non-zero) rates after 2  108 cycles. The maximum decrease, Df0,max (or normalized value Df0,max/f0,initial) is larger for larger ra, as shown in Figure 4. Another notable feature of the f0 evolution plots is the presence of sudden discontinuities (“jumps”) in f0 for larger ra. While the curves are fairly smooth for

Figure 4. Normalized maximum decrease in f0, Df0,max/f0,initial, as a function of ra. 0.1% corresponds to 8 Hz. Shaded areas summarize observed trends in fatigue damage at 30 °C, 50% RH.

low ra (see Fig. 3 for ra = 315 MPa), sudden decreases in f0 ranging from a few Hz to up to 15 Hz were observed for larger ra (see Fig. 3 for ra = 465 and 475 MPa). The above results strongly suggest that the evolution of f0 mainly results from fatigue damage accumulation at the notch of the beam. The sudden discontinuities (decreases) in f0 are likely to be associated with the formation of micro-cracks (i.e., decrease in beam’s stiffness) at the extrusions’ edges, as both sudden jumps and micro-cracks are only observed for ra above 350 MPa. The continuous decrease in f0 is interpreted as localized plastic deformation (micro-plasticity) at the notch, resulting in an overall more compliant beam, and eventually leading to extrusions and micro-cracks after sufficient numbers of cycles. The large ra (>300 MPa) needed to observe extrusions are likely to result from the large stress gradients in the films, and indicate that

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the mechanistic processes leading to fatigue crack initiation are largely influenced by stress gradients. Transmission electron microscopy (TEM) will be necessary to determine the specific nature of the dislocation arrangements underneath the extrusions associated with such strong stress gradients. The effect of environment (temperature and relative humidity) on the fatigue crack initiation process was also studied, by comparing tests at 30 °C, 50% RH, with six tests performed at 80 °C, 90% RH (ra ranging from 200 to 425 MPa). At 80 °C 90% RH, the f0 evolution is singularly different from that at 30 °C, 50% RH, consisting of an initial decrease in f0 (whose magnitude is similar to the tests at 30 °C, 50% RH; see Figs. 3 and 4), followed by an increase in f0. The increase in f0 was also observed for a specimen that was not fatigued but exposed to the environment at 80 °C, 90% RH. The increase rate in f0 for the fatigued specimens was larger for larger ra (see Fig. 3). The increase was not observed for a test performed at 80 °C, 4.5% RH, suggesting that the partial pressure of water, pw, plays a significant role (pw is equal at 30 °C, 50% RH and 80 °C, 4.5% RH). The nature and amount of fatigue damage observed along the sidewall of the notch were equivalent between 30 °C, 50% RH and 80 °C, 90% RH for similar ra, with the exception of one test at 80 °C, 90% RH (ra = 425 MPa) that showed a much higher density of extrusions and micro-cracks (along with larger Df0,max/ f0,initial value; see Figs. 2e, 3, and 4). The increase in f0 is interpreted as surface oxidation at the notch. Large elastic moduli (E = 413 GPa at 800 °C) have been calculated for oxide scales on bulk Ni, based on measured increases in f0 of long, thin cylindrical rods undergoing oxidation at large temperatures (T > 800 °C) [14]. While the presence of water vapor does not affect the oxidation rate of Ni at high temperatures [15], it is known that dissociation of water is more rapid than that of oxygen at lower temperatures [15]. It is therefore possible that the Ni thin films may form thicker surface oxides at 80 °C in the presence of large partial pressure of water and mechanical stress, based on the observed results. TEM will be used to measure the surface oxide layer thickness and its morphology for different environments and ra, and investigate its specific role on the fatigue crack initiation process.

To summarize, this paper introduced a characterization technique based on MEMS micro-resonators to investigate the VHCF behavior of electroplated Ni thin films. The films did not fail after as many as 4  109 cycles, even for ra as large as 520 MPa, a result attributed to the extreme stress gradients in these films. Hence, the effect of large stress gradients in structural thin films can be taken advantage of to design fatigue-resistant films, and should be further studied for accurate reliability predictions. The evolution of f0 during the fatigue tests bring valuable information regarding the fatigue crack initiation process of these films, including localized cyclic plasticity, micro-cracking, and possibly oxide formation in various environments. Future work will include TEM characterization to further study the formation of the observed extrusions. [1] S.Y. He, J.S. Chang, L.H. Li, H. Ho, Sens. Actuator APhys. 154 (2009) 149–156. [2] D.R. Sparks, M.I. Chia, G.Q. Jiang, Sens. Actuator APhys. 95 (2001) 61. [3] B.L. Boyce, J.R. Michael, P.G. Kotula, Acta Mater. 52 (2004) 1609–1619. [4] H.S. Cho, K.J. Hemker, K. Lian, J. Goettert, G. Dirras, Sens. Actuator A-Phys. 103 (2003) 59. [5] D. Son, J.J. Kim, J.Y. Kim, D. Kwon, Mater. Sci. Eng. A 406 (2005) 274–278. [6] Y. Yang, B.I. Imasogie, S.M. Allameh, B. Boyce, K. Lian, J. Lou, W.O. Soboyejo, Mater. Sci. Eng. A 444 (2007) 39. [7] G.P. Zhang, C.A. Volkert, R. Schwaiger, P. Wellner, E. Arzt, O. Kraft, Acta Mater. 54 (2006) 3127–3139. [8] S.E. Stanzl-Tschegg, B. Schonbauer, Int. J. Fatigue 32 (2010) 886–893. [9] C. Stocker, M. Zimmermann, H.J. Christ, Int. J. Fatigue 33 (2011) 2–9. [10] H. Mughrabi, Int. J. Fatigue 28 (2006) 1501–1508. [11] K.P. Larsen, A.A. Rasmussen, J.T. Ravnkilde, M. Ginnerup, O. Hansen, Sens. Actuator A-Phys. 103 (2003) 156. [12] O.N. Pierron, C.C. Abnet, C.L. Muhlstein, Sens. Actuator A-Phys. 128 (2006) 140–150. [13] H.W. Hoppel, J. May, P. Eisenlohr, A. Goken, Z. Metall. 96 (2005) 566–571. [14] D. Bruce, P. Hancock, J. Inst. Metals 97 (1969) 148. [15] S.R.J. Saunders, M. Monteiro, F. Rizzo, Prog. Mater. Sci. 53 (2008) 775–837.