Jae-Ik Lee

Aligned Carbon Nanotube Arrays for Degradation-Resistant, Intimate Contact in Micromechanical Devices

Figure 1 . a) Schematic of the three-terminal micromechanical switch with aligned CNT arrays as the contact material. b) Onand off-states of the contact according to the actuation of the shuttle. c) Optical microscopy images of the contact region of CNT arrays in the onand offstates according to the applied gate voltage. The CNT arrays are in contact under V G = 0 V, and the current fl ows through the contact (the on-state). At V G = 52 V, the shuttle is driven in the direction of the gate electrode and the CNT arrays are separated (the off-state). The inset shows the corresponding relationship between I SD and V G . The scale bar is 10 μ m. The emergence of and advances in nano/ microelectromechanical systems (N/ MEMS) [ 1 , 2 ] have enabled the realization of miniaturized mechanical switches with high functionality in terms of low power consumption, fast operating speed, and low cost of fabrication. Various solid contact materials have been investigated for nanoand micromechanical contacts, including dopedSi, [ 3 , 4 ] single-crystal SiC and diamond, [ 5 , 6 ] and diverse metals and their alloys. [ 7 ] However, regardless of the contact material, these solidto-solid contacts suffer from diverse failure mechanisms such as stiction by adhesion, electrical short by welding and melting, unavoidable surface degradation by mechanical wear and abrasion, and a small real area of contact due to surface asperity. Therefore, the low reliability and durability of contacts are major challenges to overcome in nanoand micromechanical switching devices. Espe-

Deformable Carbon Nanotube-Contact Pads for Inertial Microswitch to Extend Contact Time

We have demonstrated a batch-fabricated inertial microswitch with extended contact time using carbon nanotube (CNT)-contact pads. Self-assembled and aligned CNT bundles, as deformable electromechanical contact pads are selectively synthesized on fully fabricated single crystal silicon microstructures. Outstanding mechanical flexibility and resilience, as well as electrical conductivity of CNTs, make them suitable as an electromechanical contact material. It is experimentally verified that the elastic deformation of CNTs dramatically enhances the contact time of the inertial microswitch from 7.5 μs to 114 μs. Due to the prolonged contact time, the presented inertial microswitch provides reliable and easy detection of threshold acceleration, which is crucial in diverse commercial applications such as airbag restraint systems in vehicles or geriatric healthcare systems.

A highly sensitive flexible strain sensor based on the contact resistance change of carbon nanotube bundles.

A novel carbon nanotube (CNT)-based flexible strain sensor with the highest gauge factor of 4739 is presented. CNT-to-CNT contacts are fabricated on a pair of silicon electrodes fixed on a PDMS specimen for both flexibility and electrical connection. The strain is detected by the resistance change between facing CNT bundles. The proposed approach could be applied for diverse applications with a high gauge factor.

Using confined self-adjusting carbon nanotube arrays as high-sensitivity displacement sensing element.

Displacement sensing is a fundamental process in mechanical sensors such as force sensors, pressure sensors, accelerometers, and gyroscopes. Advanced techniques utilizing nanomaterials have attracted considerable attention in the drive to enhance the process. In this paper, we propose a novel and highly sensitive device for detecting small displacements. The device utilizes the changes in contact resistance between two sets of vertically aligned carbon nanotube (CNT) arrays, the growth of which was confined to enable their facile and reliable integration with fully fabricated microstructures. Using the displacement transduction of the proposed device, we successfully demonstrated a 3-axis wide bandwidth accelerometer, which was experimentally confirmed to be highly sensitive compared to conventional piezoresistive sensors. Through a test involving 1.2 million cycles of displacement transductions, the contact resistance of the CNT arrays was proved to be excellently stable, which was a consequence of the high electrical stability and mechanical durability of the CNTs.

Microswitch with self-assembled carbon nanotube arrays for high current density and reliable contact

A microswitch based on self-assembled carbon nanotube (CNT) arrays as micromechanical contact material has been demonstrated. The aligned CNT arrays are synthesized on microelectrodes and movable shuttle fabricated on a silicon-on-insulator (SOI) wafer. The CNT arrays are self-assembled on microstructures making mechanical contact between source and shuttle, and this contact is preloaded by the growth force of CNT and mechanical restoring force of the strained springs. Based on high interfacial contact area of CNT arrays, high electrical conductivity and excellent mechanical properties of CNT, highly reliable and durable micromechanical contact for microswitch is experimentally demonstrated while high-density currents are stably transmitted.

Development of flexible tactile sensor based on contact resistance of integrated carbon nanotubes

We have developed a novel three dimensional tactile sensor based on vertically aligned carbon nanotubes. The carbon nanotubes were directly synthesized on silicon microstructures and these CNTs-on-microstructures were integrated to flexible polydimethylsiloxane layers. Each tactile sensor has four sensing parts and the direction of force can be detected by monitoring the increase or decrease of electrical resistance in each sensing part. High gauge factor up to 272 and fast response less than 10 ms have been experimentally verified from the presented tactile sensor. The deviated contact resistance change from the initial value was less than 3% after repeated force input of 15 mN for 180,000 cycles.

Resonant Frequency Tuning of Torsional Microscanner by Mechanical Restriction using MEMS Actuator

We have demonstrated resonant frequency-tuning of electrostatic torsional microscanners actuated by the staggered vertical comb (SVC) sets. Mechanical restriction unit composed of thermal actuator, scissor mechanism and shaft-holder is used for continuous and reversible resonant frequency tuning. The microscanner and tuning mechanism are fabricated on single crystal silicon of double Silicon-On-Insulator (DSOI) wafer in order to make vertically self-aligned comb sets and electrically insulate the electrostatic microscanner from the thermally actuated tuning unit to prevent charge leakage. The microscanner is actuated at a resonant frequency of 1.698 kHz under driving voltages of 5Vac and 10Vdc without frequency tuning. As the stiffness of a torsional spring is modified gradually through the shaft-holding flexure operated by thermal actuator, the resonant frequency of the torsional microscanner is shifted up to 1.880 kHz showing the maximum tuning ratio of 10.7%.

Bidirectional electrothermal electromagnetic torsional microactuators for large angular motion at dc mode and high frequency resonance mode operation

This paper presents a novel design of a bidirectional torsional micromirror utilizing vertically driven electrothermal electromagnetic silicon beam actuators to generate large angular motion in both static mode and high-frequency resonance mode with low operational voltages. The microactuators are fabricated on a silicon-on-insulator (SOI) wafer using three photo masks in order to form two different thicknesses of single crystal silicon (SCS) device layer and backside cavities. When the driving bias is applied to the device in the static mode operation, four buckle beams placed alongside the torsion bars are subjected to thermal expansion and buckle in the vertical direction generating torsional displacement of the micromirror with respect to two torsion bars, the center of rotation. The direction of buckle is controlled by the Lorentz force caused by the current flowing through the silicon beams to be buckled in the magnetic field applied, enabling the bidirectional motion of the torsional micromirror. At resonance, Lorentz force itself drives the actuator instead of thermal expansion force from the buckle beams. The maximum static angular displacement of the torsional actuator is 13.42° (26.84°, optical angle) under a driving dc voltage of 7.5 V. In the resonance mode operation, the measured angular displacement is 8.22° (16.44°, optical angle) at 10.64 kHz under sinusoidal driving voltages of 0 to 4.4 V.

A novel accelerometer based on contact resistance of integrated carbon nanotubes

A novel accelerometer based on electrical contact resistance change between two sets of carbon nanotubes (CNTs) is presented. After the micro structures are fabricated by silicon bulk micromachining, the CNTs are synthesized from each of the facing surfaces of proof-mass and fixed electrode. When the motion of the proof-mass is generated from incoming acceleration, effective contact area between two sets of CNT bundles changes, resulting in a change of electrical contact resistance. This CNTs-based accelerometer achieves both high sensitivity and wide bandwidth. Two different sensing modes are experimentally verified and compared: the approach mode where the incoming acceleration pushes the proof-mass to the fixed electrode and the separation mode where the acceleration pulls away the proof-mass from the fixed electrode. In each case, the sensitivities are 7.62 mV/g/V and 17.23 mV/g/V, respectively. It is confirmed that the contact resistance remained nearly constant after 14.4 million cycles of operation under 400 Hz sinusoidal acceleration with the magnitude of −10 g ∼ +10 g.

Development of carbon nanotubes contact-based linear displacement sensor with large sensing range

A novel displacement sensor based on contact area change of two sets of vertically aligned carbon nanotubes (VACNTs) is developed. The VACNTs synthesized on movable side of two facing microstructures are in contact with another set of VACNTs on fixed side. When the movable microstructure is displaced, the VACNTs on movable side slide on those of fixed side. The measured output current signal is linear to the displacement input to the sensor. Experimentally verified sensing range is over 2 mm, and it could be easily extended or reduced by varying the overlapped length of CNTs. By monitoring two signals from two sets of facing VACNTs, it is possible to simultaneously measure both the displacement and the direction of motion. It also enables the significant reduction of possible measurement errors from the temperature fluctuation.