A new on-chip test structure for real time fatigue analysis in polysilicon MEMS G. Langfelder a, * , A. Longoni a , F. Zaraga a , A. Corigliano b , A. Ghisi b , A. Merassi c a Electronics and Information Department, Politecnico di Milano, via Ponzio 34/5, Milano, Italy b Department of Structural Engineering, Politecnico di Milano, piazza Leonardo Da Vinci 32, Milano, Italy c MAHRS Unit, STMicroelectronics, Cornaredo (Milano), Italy article info Article history: Received 21 May 2008 Received in revised form 19 November 2008 Available online 27 December 2008 abstract Fatigue test results on 15 lm thick polysilicon specimens are presented and discussed, both quantita- tively and qualitatively. The test structure is a newly designed, electrostatically actuated, MEMS device that allows the execution of on-chip fatigue and fracture tests on polysilicon specimens. The experiments have been carried out through a new analog, low-noise and low-perturbing electrostatic position mea- surement system for capacitive MEMS sensors. The setup allows for a real time monitoring of MEMS posi- tion, from which a macroscopic quantity, the elastic stiffness of the specimen, can be continuously evaluated, provided that the applied force is known. The results obtained in the present research put in evidence the decrease of the elastic stiffness during fatigue life before rupture. In addition, the stress amplitude during the load cycles plays a role on the life- time of the test devices: larger stress amplitudes around a tensile mean stress reduce the fatigue resis- tance, mainly when a compressive stress is also present, in good agreement with a Wöhler curve. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The behaviour of polycristalline silicon Micro Electro Mechani- cal Systems (MEMS) under loading cycles has been intensively studied since the early discovery of fatigue effects in this kind of structures [1]. As a brittle material, polysilicon should not be sub- jected to fatigue (continuous growth of micro-defects finally lead- ing to rupture); most of the experimental results found in the literature, however, show quite the contrary for polysilicon thin films [2–9]. These results are often quantitatively interpreted in terms of a stress–life curve, showing a decrease of fatigue life with increasing stress amplitude. Rarely a qualitative analysis on a mac- roscopic quantity varying as a consequence of microscopic dam- ages due to fatigue is performed [5]. This work aims to get an insight into fatigue in polysilicon mi- cro-structures by adopting a real time monitoring of a physical quantity, the elastic stiffness of the specimen, from which fatigue behaviour can be investigated. The experimental results have been obtained by means of a newly designed, electrostatically actuated, MEMS test device which includes an 8 lm-long notched specimen, loaded through an on-chip electrostatic actuator [10]. The position of the moving, suspended mass can be measured through an on-chip sensing capacitor. The device allows the execution of fatigue tests on poly- silicon specimens and is interfaced with an analog, low-noise and low-perturbing electrostatic position measurement system for capacitive MEMS sensors [11]. The readout electronics is based on a small amplitude and high frequency voltage signal, applied between the movable and fixed plates of the sensing capacitance. The resulting high frequency modulation allows to filter low-fre- quency noise and to minimize readout electrostatic perturbations on the device. System resolution of 100 ppm at 10 Hz effective bandwidth has been obtained. Test results on 15 lm-thick polysilicon specimens are pre- sented: first, the elastic stiffness evolution is analysed on a qualita- tive level and then the collected failure data are compared on a quantitative level. The setup guarantees a continuous monitoring of the MEMS position, from which the elastic stiffness evolution of the specimen can be acquired in real time, as the constant ap- plied force is known. Before the rupture, the elastic stiffness de- crease can be related to microscopic damage growth. After the rupture, data are collected into stress–life (S–N) plots and are dis- cussed with reference to the ones found in the existing literature [2,8,9]. 2. The test structure The MEMS structures used in this work are fabricated with the STMicroelectronics THELMA Ó process and, as depicted in Fig. 1, are formed by a rectangular shaped seismic, suspended mass forming with the constrained parts two sets of comb finger capacitors: the former serving as electrostatic actuator, the latter serving as sensor capacitance [10]. The seismic mass includes, in its lower part, a le- ver system which delivers the actuator force to a notched beam. 0026-2714/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2008.11.009 * Corresponding author. Tel.: +39 0223993425; fax: +39 022367604. E-mail address: giacomo.langfelder@polimi.it (G. Langfelder). Microelectronics Reliability 49 (2009) 120–126 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel