**1. Introduction**

434 Microelectromechanical Systems and Devices

Torczynski, J. R.; Gallis, M. A.; Piekos, E. S.; Serrano, J. R.; Phinney, L. M. & Gorby, A. D.

Wu, B.; Kumar, A. & Pamarthy, S. (2010) High Aspect Ratio Silicon Etch: A Review. *Journal* 

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(2008) Validation of Thermal Models for a Prototypical MEMS Thermal Actuator, Sandia Report SAND2008-5749, Sandia National Laboratories, Albuquerque, NM Wong, C. C. & Phinney, L. M. (2007) Computational Analysis of Responses of Micro Electro-

Thermal Actuators, *Proceedings of ASME 2007 International Mechanical Engineering Congress & Exposition* Paper No. IMECE2007-41462, 9 pp., Seattle, Washington,

> The microelectronics industry has been consistently driven by the scaling roadmap, colloquially referred to as the Moore's law. Consequently, during the past decades, integrated circuits have scaled down further. This shrinkage could have never been possible without the efficient integration and exploitation of thin film materials.

> Thin film materials, on the other hand, are the essential building blocks of the micro- and nano-electromechanical systems (MEMS and NEMS). Utilization of thin film materials provides a unique capability of further miniaturizing electromechanical devices in microand nano-scale. These devices are the main components of many sensors and actuators that perform electrical, mechanical, chemical, and biological functions. In addition to the wide application of thin film materials in micro- and nano-systems, this class of materials has been historically utilized in optical components, wear resistant coatings, protective and decorative coatings, as well as thermal barrier coatings on gas turbine blades.

> In some applications, thin film materials are used mainly as the load-bearing component of the device. Microelectromechanical systems (MEMS) are the example of these applications. Thin film materials carry mechanical loads in thermal actuators, switches and capacitors in RF MEMS, optical switches, micro-mirror hinges, micro-motors, and many other miniaturized devices. In these applications, one of the main criteria to choose a specific material is its ability to perform the mechanical requirements. Therefore, a clear understanding of the mechanical behavior of thin film materials is of great importance in these applications. This understanding helps better analyze the creep in thermal actuators (Tuck et al., 2005; Paryab et al., 2006), to investigate the fatigue of polysilicon (Mulhstein et al., 2001; Shrotriya et al., 2004) and metallic micro-structures (Eberl et al., 2006; Larsen et al., 2003), to scrutinize the relaxation and creep behavior of switches made of aluminum (Park et al., 2006; Modlinski et al., 2004) and gold films (Gall et al., 2004), to study the hinge memory effect (creep) in micro-mirrors (Sontheimer, 2002), and to address the wear issues in micro-motors. (van Spengen, 2003)

> In some other applications, the thin film material is not necessarily performing a mechanical function. However, during the fabrication process or over the normal life, the device experiences mechanical loads and hence may suffer from any of the mechanical failure issues. Examples of these cases are the thermal fatigue in IC interconnects (Gudmundson & Wikstrom, 2002), strain ratcheting in passivated films (Huang et al., 2002; He et al., 2000), the

Standalone Tensile Testing of Thin Film Materials for MEMS/NEMS Applications 437

Nano-indentation is the advanced version of the classical hardness test method. In this technique, the specimen is loaded by a sharp indenter and the load-displacement (*P-h*) of the indenter is monitored during loading and unloading. The reduced elastic modulus and hardness are the two material parameters that can be extracted from the *P-h* data. This

Curvature method is one of the early methods that was used to probe the mechanical behavior of the thin film materials on substrate. In this method, the initial curvature of a substrate is measured and then the film material is deposited on the substrate. The variations in the curvature of the substrate before and after the deposition of the film are a good measure of the residual stresses in the film. This method can also be used to investigate the mechanical behavior of thin film materials on substrate under temperature

Among the aforementioned experimental methods, tensile testing technique is the only technique that can be used to extract the mechanical behavior of thin film materials under different loading conditions. In this method, all material parameters can be directly measured from the experimental data and it provides a straight-forward approach to the measurement. However, this method faces its own challenges in sample preparation, handling, and gripping and involves uncertainties in the measured strains. In the following sections, these challenges are discussed and different approaches to tackle these problems

The early efforts in the tensile testing of thin films were the concurrent research work of Ruud et al., 1993, Koskinen et al., 1993, and Read & Dally, 1993 in the early 1990's. Ruud et al., 1993 introduced a tensile testing technique to test free standing thin film specimens with gage section area of 10 mm long by 3.3 mm wide. They sandwiched the specimen ends between polished aluminum grippers using 5µm thick copper films and used a motordriven micrometer for loading. Strain was measured by monitoring the displacement of laser spots diffracted from a series of lithography patterned photoresist islands. With this technique, they managed to determine the Young's modulus, Poisson's ratio, and yield strength of free-standing Cu, Ag, and Ni films (Ruud et al., 1993) and Ag/Cu multilayers (Huang & Spaepen, 2000), and to study the yield strength (Yu & Spaepen, 2004) and

Koskinen et al., 1993 used a relatively simple technique to test LPCVD polysilicon films. They introduced a gripping setup that could hold an array of 20 samples and was capable of loading individual specimens. Specimen ends were glued to the gripper and loaded by a motor driven stage. Gripper displacement was measured and used to calculate the strain. While both of these techniques suffered from a reliable gripping and load train alignment, Read & Dally, 1993 and Read, 1998a, 1998b developed a sample fabrication procedure that could meet the demanding gripping and alignment issues, simultaneously. In their method, films were deposited on silicon substrate and after patterning the film to a dog-bone shape, the substrate was etched from the backside to open a window frame under the film, leaving it free-standing. After mounting the specimen in grippers, the frame edges were cut so that only the film is carrying the load. In this way, since the thick substrate is mounted in gripper jaws, there would be less slip and alignment will be an easier task. The concept of free standing film on supporting frame was used by other researchers to overcome the

method can only be used for thin films on substrate.

**3. Tensile testing techniques for thin films** 

anelastic behavior (Yu, 2003) of thin Cu films on Kapton substrate.

cycling.

are presented.

fracture and delamination of thin films on flexible substrates (Li & Suo, 2006), the fracture of porous low-k dielectrics (Tsui et al., 2005), electromigration (He et al., 2004), the chippackage-interaction (CPI) (Wang & Ho, 2005), and thin film buckling and delamination (Sridhar et al., 2001).

In order to address the above-mentioned failure issues and to design a device that has mechanical integrity and material reliability, an in-depth knowledge of the mechanical behavior of thin film materials is required. This information will help engineers integrate materials and design devices that are mechanically reliable and can perform their specific functions during their life-time without any mechanical failure.

In addition to the tremendous industrial and technological driving force that was mentioned earlier, there is a strong scientific motivation to study the mechanical behavior of thin film materials. The mechanical behavior of thin film structures have been known to drastically differ from their bulk counterparts. (Xiang, 2005) This discrepancy that has been referred to as the length-scale effect has been one of the main motivations in the scientific society to study the mechanical behavior of thin film materials. In order to provide fundamental mechanistic understanding of this class of materials, old problems and many of the known physical laws in materials science and mechanical engineering have to be revisited from a different and multidisciplinary prospective. These investigations will not be possible unless a concrete understanding of the mechanical behavior of thin film materials is achieved through rigorous experimental and theoretical research in this area.
