Wafer Level Mechanical Testing of Al Films at High and Low Temperature

There has been growing interest to develop a reliable and repeatable method for investigating the mechanical properties of thin films and MEMS materials. In this study, the mechanical response of suspended thin film Al membranes are examined by the Membrane Deflection Experiment (MDE) and nanoindentation. The MDE tests were conducted at temperatures of 2, 27, and 60°C in order to ascertain differences in the membrane response with temperature. The MDE tests were found to yield E, σr, and σy for the membranes while the film thickness and roughness obstructed interpretation of the nanoindentation data. INTRODUCTION Thin films are widely employed as components of MEMS devices. Their mechanical properties frequently govern the functional aspects of the device and therefore directly influence its reliability. Determining the mechanical properties is essential in designing commercially viable MEMS devices. These devices will likely have to operate under differing temperature regimes and thus, mechanical properties must be determined as a function of temperature. Nanoindentation is a means to determine Young’s modulus, E, and hardness, H, at small scales specimens [1-4]. However, when film thickness becomes small, < 1 μm, the indenter can only probe to depths of about 100 nm before substrate effects begin to influence the results [3,5-7]. Another aspect to consider is the surface roughness of the film. Typically, the indenter must probe to a depth of approximately 10 times the surface roughness to ensure that the indenter contact area is nominal [7,8]. For the case of a 1 μm thick film, roughness must be < 10 nm. In many thin film materials, these conditions are difficult or even impossible to achieve. A novel wafer level mechanical testing scheme involving the deflection of suspended thin film membranes was developed by Espinosa et al. [9-12]. The procedure involves applying a line-load, with a nanoindenter, at the center of the spanning membrane to measure its mechanical response. This method will be used to determine E, σr, and σy at temperatures of 2, 27, and 60°C. The method and results will be compared with that of nanoindentation on the same material. EXPERIMENTAL PROCEDURE Specially designed thin film Al specimens, used in the design of RF-Switches [9,12] were microfabricated in collaboration with Raytheon Company. A specimen shape was chosen to allow testing with the Membrane Deflection Experiment (MDE) and to subject the specimens gauged region to pure tension [10-11]. A schematic of the specimen geometry is shown in Fig. 1 and the corresponding dimensions for 4 differently sized specimens are listed in Table 1. The membrane is supported by two posts, such that they are suspended approximately 4 μm above the substrate. Specific dimensions were chosen in order to achieve plasticity at maximum deflection. The MDE test is a method for testing the mechanical response of freestanding thin film specimens. The procedure involves applying a line-load, with a nanoindenter, to the center of the spanning membrane. Load is applied and the membrane is deflected downwards until it makes contact with the substrate. The result is direct tension in the gauged regions of the membrane with load and deflection being measured independently, see Fig. 2. Fig. 1. Schematic representation of the aluminum membrane geometry. Table 1. Dimensions of Suspended Membranes Sample L (μm) r (μm) W (μm) WL (μm) 1 100 10 10 10 2 100 10 20 10 3 50 5 10 5 4 50 5 5 5