A micromechanical switch with electrostatically driven liquid-metal droplet

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Abstract

This paper presents the use of a microscale liquid-metal droplet as a contact and moving part in a micromechanical switch with electrostatic actuation. Design, FEM analysis, fabrication and testing of the device are reported. The droplet is driven by a given voltage bias that induces electrostatic force between a grounded liquid-metal and an imbedded actuation electrode. The electrodes and the liquid-metal droplet are placed inside of an anisotropically etched silicon cavity. A novel technique to make shadow masks utilizing thin wafers is used to pattern the electrodes inside the silicon cavity.

Introduction

Due to functional advantages of micromechanical switches over transistor switches, various types of micromechanical switches have been introduced [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] since micromachining technologies were pioneered in 1980s. These micromechanical switches show a high on–off impedance ratio, wide operating temperature range, and radiation insensibility. Generally, they can also handle high voltages and currents. However, almost all micromechanical switches are designed with solid-to-solid contacts that exhibit the same problems as macroscale mechanical switches, such as contact surface degradation and signal bounce. These problems are magnified for microswitches due to the increased importance of contact surfaces in microscale.

In order to solve the reliability problems of the solid-to-solid contact, mercury is used in some high-end electronic instruments. Mercury’s low contact resistance and its lack of signal bounce and contact wear greatly increase the instruments’ performance. Moreover, in microscale, mercury droplets show a great physical stability since inertial forces become negligible while surface tension force gains its importance.

To utilize from the above properties of mercury, a gap closing microcantilever microswitch that used a stationary mercury microdroplet at the point of contact (Fig. 1a) and a thermally driven mercury microswitch (Fig. 1b) were developed and tested [11], [12]. Although these devices demonstrated successful operation, they are limited by the size of their actuation sources (i.e., beam length and heater size) as well as large power consumption (on the order of mW for thermal actuation). In order to design a microswitch with reduced size, low power consumption, and simple fabrication, electrostatic actuation of the droplet is considered with the mercury droplet as a moving part, following a recent theoretical study [13]. Different applications of electrostatic actuation with liquid droplets (not with liquid-metal droplets) on a solid surface have been introduced in recent literature [14], [15].

By combining the reliability and high quality of a mercury contact along with the simple implementation of electrostatic actuation, we are developing micromechanical switches that are small enough to be considered for memory chips. Our current project goal is integration of the micromechanical switch arrays directly on top of CMOS chips to realize reconfigurable circuits for space applications [16].

Section snippets

Theoretical approach

Let us consider a liquid droplet resting on top of a solid surface (Fig. 2). Assuming that the droplet is small enough so that gravitational forces can be neglected, the droplet geometry is a truncated sphere fully characterized by two parameters: the droplet radius R and the contact angle θ. The contact angle is a thermodynamic property that depends on the interfacial energies among three materials (liquid/vapor γLV, solid/vapor γSV, and liquid/solid γLS) as described in the Young’s equation

Device design and FEM analysis

In order to confine the droplet and simplify subsequent packaging, the current device (Fig. 6) has been designed inside of a bulk-etched cavity on a silicon wafer. The droplet is designed to move to the directions of the arrow. The two actuation electrodes are embedded underneath a dielectric layer, and the ground electrode and the two signal electrodes are on top of the dielectric layer. For the droplet, the smaller circle drawn with broken lines indicates the area where the droplet is in

Shadow mask fabrication

For our current device design, metal electrode lines need to be patterned inside of the Si cavity (bottom surface and side walls). In order to pattern metal lines less than 30 μm in width inside a cavity that is about 100 μm deep, we developed a new shadow mask technique using thin silicon wafers (∼40 μm), which simplifies the processing and provides higher resolution compared with previously reported patterning methods [19]. To handle such a fragile thin wafer, a glass wafer (Borofloat™), about

Results and discussion

Fig. 12 shows a completed device, corresponding to the schematics in Fig. 6. The mercury droplet is grounded through the ground electrode. Then a voltage is applied to one of the imbedded actuation electrode, attracting and sliding the droplet. By switching the applied voltage to the other actuation electrode, the droplet is attracted back to its initial position, so the signal circuit can be turned on and off.

Fig. 13 shows “off” and “on” positions of the microswitch. The device is physically

Conclusions

A new type of microswitch, which employs electrostatic actuation of a liquid-metal droplet, has been introduced. The device is small enough to let us consider applications in reconfigurable circuits or high-density micromehanical memories. We use silicon bulk etching and a new shadow mask technology to fabricate our devices. Actuation has been achieved in a viscous oil with a relatively high voltage of 100–150 V at a switching frequency of 1 Hz. Currently, we are exploring several ways to control

Acknowledgements

The authors would like to thank P. de Guzman for his assistance in the design and building of the optical alignment equipment. Part of the contact angle measurement has been done in the EQE division of the French Space Agency (CNES, Electronic Analysis Laboratory) thanks to F. Pressecq and F. Courtade. This work has been supported by CNES and the NSF CAREER Award (ECS-9702875).

Joonwon Kim received the MS and BS degree in mechanical engineering from University of California at Los Angeles (UCLA), in June 1999 and March 1997, respectively. Currently, he is working toward his PhD degree in mechanical engineering majoring in microelectromechanical systems (MEMS) at UCLA with an Exceptional Student Admission Program (ESAP). His current research topics are related to liquid-metal applications and surface and contact properties of MEMS applications. He is also interested in

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Joonwon Kim received the MS and BS degree in mechanical engineering from University of California at Los Angeles (UCLA), in June 1999 and March 1997, respectively. Currently, he is working toward his PhD degree in mechanical engineering majoring in microelectromechanical systems (MEMS) at UCLA with an Exceptional Student Admission Program (ESAP). His current research topics are related to liquid-metal applications and surface and contact properties of MEMS applications. He is also interested in optical MEMS, bioMEMS and MEMS design and testing.

Wenjiang Shen received the BS and MS degree in the materials science and engineering from Tsinghua University, PR China, in 1997 and 2000, respectively. He joined the UCLA Micromanufacturing Lab. in August 2000. He is currently working on liquid-metal droplet microswitches.

Laurent Latorre received the “Diplôme d’ingénieur” from l’Ecole Nationale d’Ingénieurs de Belfort in 1994. He received the “Diplome d’Etudes Approfondies” and the PhD degree in microelectronics from the University of Montpellier in 1995 and 1999, respectively. His research at the LIRMM laboratory concerns integrated circuits design and microsystems.

Chang-Jin Kim received the PhD degree in mechanical engineering from the University of California at Berkeley in 1991. He received the BS degree from Seoul National University and MS from Iowa State University along with the Graduate Research Excellence Award. Upon joining the faculty at UCLA in 1993, he has developed several MEMS courses and established a MEMS PhD major field in Mechanical and Aerospace Engineering Department. His research is in MEMS and nanotechnology, including design and fabrication of micro/nano structures, actuators and systems, with a recent focus on the use of surface tension. Prof. Kim is the recipient of the 1995 TRW Outstanding Young Teacher Award and the 1997 NSF CAREER Award. Prof. Kim served as a Chairman of the Micromechanical Systems Panel of the ASME DSC Division in 1996 and co-organized the MEMS Symposia between 1994 and 1996 for the ASME International Mechanical Engineering Congress and Exposition. He also organized the 1996 ASME Satellite Broadcast Program on MEMS. He served as General Co-chairman of the Sixth IEEE International Conference on Emerging Technologies and Factory Automation and served in the Technical Program Committees of the IEEE MEMS Workshop (1998) and the SPIE Symposium on Micromachining and Microfabrication (1998–2000). Currently he is serving in the US Army Science Board, the Technical Program Committee of Transducers’01, the Executive Committee of ASME MEMS Subdivision, and as a Subject Editor for the IEEE/ASME Journal of MEMS.

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