Methods to Measure Mechanical Properties of NEMS and MEMS: Challenges and Pitfalls
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1185-II04-06
Methods to Measure Mechanical Properties of NEMS and MEMS: Challenges and Pitfalls Ingrid De Wolf1,2, Stanislaw Kalicinski1,2, Jeroen De Coster1, Herman Oprins1 1 IMEC, Kapeldreef 75, Leuven, B-3001 Belgium, 2 K.U.Leuven, Dept. MTM, Leuven, B-3001 Belgium,
ABSTRACT This paper focuses on the measurement of material properties of micro and nanoelectromechanical systems. Two different methods are discussed: electrical or optical measurements of the resonance frequency, and measurements of the Raman frequency shift. The main focus of this paper is on challenges and pitfalls related to the use of these techniques for the study of MEMS and NEMS.
INTRODUCTION The main difference between micro- and nano electro-mechanical systems (MEMS, NEMS) and ICs is that the former can (or have to) move. Because of this motion, mechanical issues such as creep, fatigue, fracture, stress, stress gradient, yield strength etc. play a major role in determining their functionality and reliability. Also the ambient in which this motion takes place (normal air, vacuum, N2 etc.), the temperature, the cleanness of this ambient (particles, outgassing, adsorption) plays a major role and can have a direct effect on the mechanical properties of these devices. Measuring these properties is not an easy task. First of all, the assumption that the mechanical properties of a film are the same for the NEMS/MEMS made-out of this film is not necessary correct. Secondly, to test for example for fatigue, one should be able to monitor these systems during a very long time, while stress-cycling them. This is in general relatively easy if a fatigue sensitive parameter (for example the resonance frequency of a Si -resonator) can be measured through electrical means. It becomes more complicated if it has to be measured through optical means. Thirdly, drifts in resonance frequency, a parameter that is often used to check whether there are mechanical reliability issues, can also be caused by other mechanisms, such as charging, outgassing, adsorption, fly catching effect etc. Especially on the nano scale, this becomes important. How can it be ensured that the envisioned mechanical property is being monitored? And last, the risk that the measurement system interferes with the mechanical property to be measured is increasing with decreasing dimensions of the devices. In this paper we will address these issues and propose possible solutions to the problems discussed.
RESONANCE FREQUENCY Electrical measurements The resonance frequency of NEMS or MEMS is often used to monitor possible changes in mechanical properties. Changes in resonance frequency were for example used in [1] to study fatigue in poly-crystalline Si MEMS using dedicated test structures. The vibration frequency can also change due to mass increase or decrease, for example caused by outgassing or absorption processes [2]. In addition, this can also be due to the so called ‘fly catching effect’ [3, 4]: Electrostatic charging of NEMS or MEMS can occur because of high frequency vibrations in a
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