Measuring time-dependent deformations in metallic MEMS - Materials ...

Microelectronics Reliability 51 (2011) 1054–1059

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Measuring time-dependent deformations in metallic MEMS L.I.J.C. Bergers a,b,c, J.P.M. Hoefnagels a,⇑, N.K.R. Delhey a, M.G.D. Geers a a

Eindhoven University of Technology, Department of Mechanical Engineering, Materials Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Foundation for Fundamental Research on Matter, P.O. Box 3021, 3502 GA Utrecht, The Netherlands c Materials innovation institute M2i, P.O. Box 5008, 2600 GA Delft, The Netherlands b

a r t i c l e

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Article history: Received 24 October 2010 Received in revised form 26 February 2011 Accepted 6 March 2011 Available online 27 March 2011

a b s t r a c t The reliability of metallic microelectromechanical systems (MEMS) depends on time-dependent deformation such as creep. Key to this process is the interaction between microstructural length scales and dimensional length scales, so-called size-effects. As a first critical step towards studying these size-effects in time-dependent deformation, a purely mechanical experimental methodology has been developed, which is presented here. The methodology entails the application of a constant deflection to a lm-sized free-standing aluminum-alloy cantilever beam for a prolonged period of time. After this load is removed, the deformation evolution is immediately recorded by acquiring surface topographies through confocal optical profilometry. Image correlation and an algorithm based on elastic beam theory are applied to the full-field beam profiles to correct drift and improve limited optical profilometry precision, yielding the tip deflection as function of time with a precision of 7% of the surface roughness. A proof-of-principle measurement reveals a remarkable time-dependent deflection recovery. Assumptions and errors of the methodology are analyzed. Finally, it is concluded that the methodology is most suitable for the investigation of creep due to the simplicity of specimen handling, preparation and setup design, while maximizing long term stability and deformation precision. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Metals as structural components in MEMS find increasing applications, e.g., in radio-frequency MEMS (RF-MEMS) intended for mobile communications applications due to their favorable electrical characteristics. Fig. 1 shows an example of an RF-MEMS switch. The reliability of these devices has been shown to critically depend on their time-dependent mechanics, such as fatigue and creep, which are affected by temperature [1–4]. Fatigue affects the device life time through its limitation on the number of device operation cycles, e.g., the number of open/closed cycles of an RF-MEMS switch. Creep can directly affect the operational characteristic, e.g. through a shift in pull-in voltage of an RF-MEMS switch which results in reduced power handling [4]. Fatigue effects may pose less of a problem than expected at small geometrical length scales, whereas creep effects seem to impose more limitations upon miniaturization [5]. The difference between micro- and macroscale creep is generally attributed to so-called size-effects: the interaction between microstructural length scales and dimensional length scales [6,7]. Although for bulk materials the physical micro-mechanisms of creep are well understood, for thin films this is not the case. Specifically for free-standing metallic thin films, not much research has ⇑ Corresponding author. Tel.: +31 40 247 5894. E-mail address: [email protected] (J.P.M. Hoefnagels). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.03.008

focused on characterizing size- and temperature-effects in timedependent material behavior [8], although some research has been conducted on the influence of alloy content and structure on creep in thin metallic films [9,10]. Therefore, there is a clear need for detailed studies into the physical micro-mechanisms underlying temperature dependent size-effects in creep in metallic MEMS. As a first step in this direction, the goal of the current work is to construct and validate an experimental mechanical methodology to quantify time-dependent deformation of lm-sized free-standing cantilever beams, which can also be used at elevated temperatures up to 200 °C. To this end, first the experimental methodology is discussed in terms of design choices and data analysis steps. Next, the details of the experimental setup and implementation of the methodology are presented. Subsequently, results of proof-of-principle measurements are presented followed by a critical discussion on the precision of the methodology. To this end, the influence of various errors on the precision is evaluated, where possible quantitatively. Finally, conclusions are drawn with respect to the results of this work.

2. Design of experiment Performing mechanical tests on specimens that are free-standing with dimensions in the order of lm’s is not trivial. Aspects of specimen preparation, handling, loading, load and deformation

L.I.J.C. Bergers et al. / Microelectronics Reliability 51 (2011) 1054–1059

Fig. 1. Scanning electron microscopy image of an RF-MEMS switch (courtesy of EPCOS Netherlands B.V.).

measurement and control have to be carefully addressed [11] and will be considered in the following. 2.1. Sample handling and preparation The prolonged out-of-plane bending of the hinges in the actual MEMS device raises reliability concerns. Therefore, bending is the deformation mode of interest. Suitable test structures for bending under well defined conditions are free-standing, micron scale cantilever beams, see Fig. 2. Considering specimen handling and preparation, it is highly preferred to test on-wafer structures instead of seperate lm-sized structures. Moreover, applying the same microfabrication procedure as done for the actual device guarantees the relevance of obtained results. 2.2. Loading method To characterize parameters in microbeam bending, such as Young’s modulus or flow/fracture behavior, several methodologies exist using external/instrumented actuation [12–14] or even onchip integrated actuation [15–17]. However, a critical aspect of the load and displacement control and measurement is long term stability, especially for prolonged time-dependent deformation

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measurements (at elevated temperature). Therefore, a small, simple fully-mechanical, deflection-controlled mechanism is most suitable. To this end, a so-called micro-clamp is designed, being a simple horizontal knife edge attached to an elastic mechanism, see Fig. 3. A chip with cantilevers is placed under the knife edge, which can be lowered by rotating the thumbscrew, thus pressing and deflecting a cantilever. By using a mono-material, near-monolithic, elastic mechanism the knife edge’s vertical position can be controlled to