Quasi electrical contact behaviors of metal micro-contacts for micro/nano-electromechanical systems (MEMS/NEMS)
Date of Issue2015
School of Electrical and Electronic Engineering
The electrical contact behavior of metal micro-contacts is of significant importance in various fields of science and engineering, and it has not yet been adequately understood compared to the bulk metal contacts. Specifically, in DC contact type of microelectromechanical (MEMS) or nanoelectromechanical (NEMS) devices, the metal contact is one of the most crucial parts, as it determines the device performance and is closely related to the reliability issues. In order to investigate the degradation mechanism of the metal contacts under different testing conditions, load cycling tests have been performed using MEMS switches. The micro-contact behavior during a single load test has also been intensively studied. The electrical contact resistance (Rc) as a key parameter was characterized and analyzed to understand the contact behavior. In general, the relation between Rc and contact force Fc can be divided into three regions for a typical contact cycle. It starts from an unstable contact region (Region I) with drastic fluctuations of Rc, followed by a stable but gradual reduction of Rc in Region II and reaches a steady state with low Rc in Region III. A minimum contact force Fmin is required to establish the stable electrical contact. When the contact force exceeds Fmin (in Region II), the gradual reduction of Rc could be attributed to plastic deformation of surface asperities until the high force region (Region III), in which Rc is determined by film thickness effects on a macroscopic scale. The metal-to-metal contact behaviors have been widely studied by many different research groups. However, the unstable electrical contact behavior under low contact force (Region I) remains unexamined. In conventional MEMS switches, the contact force is believed to be larger than Fmin for most cases. Therefore, past studies of contact behavior mainly focused on the stable region, including the load cycling tests. On the other hand, there is an ongoing demand to scale down the MEMS components towards sub-micrometer and nanometer dimensions, in which the contact force could be drastically reduced. Therefore, it becomes necessary to look into the unstable contact region of metal micro-contacts. In this thesis, the electrical contact instability is investigated. A nanoindentation stage was utilized for the contact tests under precisely controlled experiential conditions (various combinations of applied voltage, contact material, surface treatment, etc.). X-ray photoelectron spectroscopy (XPS) techniques were used to characterize the sample surface. The results reveal that the contamination film on the metal contact surface, which may originate from the residues of microfabrication process or absorption of air-borne species such as hydrocarbons and carbon dioxide, plays a critical role in determining the micro-contact behavior under low contact force. The rapid fluctuations of contact resistance in the unstable region could be explained under the framework of electron tunneling through the alien contamination film. The asperity deformation process, which was conventionally observed in the stable region, is also observed at the initial stage of contact formation. To better understand the unstable contact behavior, the electrical conduction noise of DC contact type MEMS switches has been analyzed. In the power spectral density (PSD) curve of the electrical conduction noise, Lorentzian and 1/f 2 components are clearly identified. The 1/f 2 noise can be attributed to the inhomogeneous local conductivity of the thin gold film, while the Lorentzian spectrum is closely related to the electrical contact instability in the unstable contact region. The contact voltage fluctuation shows a typical two-level RTS behavior with “on” and “off” states, which could be caused by the trapping and detrapping of electrons in the alien film. This is in good agreement with the Lorentzian component in the low frequency noise (LFN) spectrum. Moreover, the relaxation time extracted from the Lorentzian component is used to determine the trap density. The electrical contact formation process between metal contacts with an alien film was further investigated in detail with statistical means. It is unveiled that the electrical conduction in the contamination film follows the weakest-link principle. The occurrence of the switching event (fluctuation between “on” and “off” states) has an independent character, till the generation of certain “weak” spots close to the end of the unstable region. The electrical contact formation process is found to be electrical field enhanced and sensitive to the thickness of the contamination film by Weibull analysis. A geometrical picture could be used to describe the breakdown of the insulating alien film, in which the stable electrical contact is eventually established by overlapping “weak” spots that form a percolation path between the micro-contacts. The results provide further insights into the contact formation process in the MEMS/NEMS devices. In addition, a comparative contact study was conducted between Au-to-Au and Ru-to-Ru contacts, to investigate the effects of various stressing conditions on the quasi electrical contact behaviors. Time-to-stable contact formation tSCF is shown to be an effective indicator of the surface conditions. It is found that Au-to-Au contact is relatively stable upon cycling under the force level of conventional MEMS switches, as the contamination film on the contact surface is rather persistent. On the other hand, Ru-to-Ru contact is sensitive to the loading cycles as well as the applied voltage during cycling, which could be due to the partially filled d-band electron structures of Ru, and the surface chemical reactions related to localized joule heating at the contact spot. A new and effective in situ approach is demonstrated to study the degradation of metal contact surface during cycling, which is of great importance for the reliability of DC type of MEMS/NEMS devices.
DRNTU::Engineering::Electrical and electronic engineering::Microelectromechanical systems