Optimization of mechanical properties of thin free-standing metal films for RF-MEMS
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Optimization of mechanical properties of thin free-standing metal films for RF-MEMS Jaap M.J. den Toonder1,2 and Auke R. van Dijken1 1 Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands. 2 Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands. ABSTRACT The mechanical properties of the thin film materials used in RF-MEMS are crucial for the reliability and proper functioning of the devices. In this paper we study a large number of aluminum alloys as possible RF-MEMS thin film materials. The yield strength and creep properties are measured using nano-indentation. The results show that the mechanical properties of thin aluminum films can be improved substantially by alloying elements. Of the alloys studied in this paper, AlCuMgMn in particular seems quite promising as a thin film material for RF MEMS, having both high yield strength and little creep. Using X-ray diffraction and electron microscopy, the observed effects are partly explained. INTRODUCTION Within Philips Research we are developing Micro Electro Mechanical Systems (MEMS) for Radio Frequency (RF) functions in wireless applications. Examples of particular functions that are being developed are tuneable capacitors and switches. There are several advantages of RFMEMS with respect to the conventional passives and switches: for example, they provide critical reductions in power consumption and signal loss thereby extending battery life or reducing weight, and they can be integrated on-chip which enhances miniaturization. An example of an RF-MEMS is the tuneable capacitor shown in Figure 1. This clearly shows the main feature of an RF-MEMS, which is a thin free-standing structured metal film with a thickness of 5 µm, attached to the substrate at a number of anchor points. The typical lateral size of the component is on the order of 100 µm. There is a gap of about 1 µm between the metal film and the substrate. A second metal electrode is buried in the substrate, and by applying a voltage difference between this electrode and the free-standing metal film, the latter is actuated by the resulting electrostatic force. In this way, the gap is reduced and the capacitance can be tuned in a controlled way. The RF-MEMS are made using dedicated processes as described in van Beek et al. [1]. top electrode
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bottom electrode
Figure 1. SEM photograph of a tuneable MEMS capacitor.
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The mechanical properties of the thin film materials used as the free-standing part in RF MEMS are crucial for the reliability and proper functioning of the devices. What is required, in particular, are a sufficiently large yield strength and a low sensitivity to creep to avoid permanent deformation of the films both during processing and use. Our first attempts to make the MEMS were done with sputtered aluminum. Due to the low yield strength and poor creep properties of aluminum, however, those first components were mostly heavily deformed plastically during processing. It is well-known from the literature on bul
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