Ultra-low voltage MEMS switch using a folded hinge structure
© Kim et al.; licensee Springer 2014
Received: 7 February 2014
Accepted: 28 April 2014
Published: 17 June 2014
An ultra-low voltage microelectromechanical system (MEMS) switch for low-power integrated circuit (IC) applications is proposed, fabricated and demonstrated. The folded hinge structure allows a large beam structure to be suspended with a designed air gap, effectively suppressing unwanted deflection. The actuation voltage of the switch was measured to be 1.7 V, the lowest among electrostatic switches. There was no variation in the actuation voltage until 106 cyclic actuations, showing the stability of a low actuation voltage in electrostatic actuation for the first time. The contact resistance was around 12 Ω, caused by a low contact force below 1 μN despite an Au–Au contact.
Microelectromechanical/nanoelectromechanical system (M/NEMS) switches have attracted attention as suitable alternatives to metal oxide semiconductor field effect transistors (MOSFETs) owing to their extremely low leakage currents [1, 2]. However, the high actuation voltages (i.e. pull-in voltage) of mechanical switches have been an obstacle for them to be deployed diversely and widely in real applications. Piezoelectric actuation [3, 4] might be one solution to lower the actuation voltage; however, the switching energy of the piezoelectric actuation is larger than that of electrostatic actuation . Moreover, materials exhibiting substantial piezoelectricity can only be applied.
The actuation voltage for electrostatically actuating mechanical switches is mainly governed by the mechanical stiffness of the spring and the size of the air gap. Fabricating mechanical switches with a low stiffness and a thin air gap is challenging because a mechanically compliant structure is subject to bending because of stress and stress gradients in the suspended beam.
We recently reported an electrostatically actuating mechanical switch that was actuated at 3.0 V, and its application in mechanical logic gates . Though the actuation voltage is the lowest among electrostatically actuating MEMS switches, it was still higher than the designed value because of an increased thickness in the air gap, caused by the mechanical stress of the electroplated hinge structure. Here we present an ultra-low voltage mechanical switch actuating at 1.7 V by introducing a folded hinge structure, effectively suppressing the deflection in the compliant hinges. Additionally, the actuation voltage variation during cyclic operations was recorded experimentally with a low actuation voltage level, for the first time.
The contact resistance was much lower in comparison with that in our earlier work  (1–2 kΩ) where the contact was formed with Au-to-Ni. A higher contact resistance increases the electrical delay in low-power integrated circuit (IC) applications. As the mechanical delay of the switches was much larger than the electrical delay, the allowed level for the contact resistance was around several kilo ohms [13, 14]. The measured contact resistance was far below the ideal level.
A MEMS switch electrostatically actuated at 1.7 V was successfully demonstrated. Introducing folded hinges was the key to achieve the designed air gap in the suspended beam with almost no deflection. The contact resistance for the fabricated switches was around 12 Ω, which is higher than expected because of an additional film residing on the surface of the contact. This film was mainly composed of carbon and oxygen, identified by SIMS analysis, which is considered as a source for the high contact resistance. The fabricated switches operated well up to 106 cycles, without a noticeable change in the actuation voltage. We demonstrated the stability of a low actuation voltage in electrostatic actuation for the first time. To the best of our knowledge, the actuation voltage of 1.7 V is the lowest voltage among electrostatic MEMS switches.
This work was supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project (CISS -2012M3A6A6054187).
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