Integration of Carbon Nanotube Network on Microelectromechanical

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Functional Nanostructured Materials (including low-D carbon)

Integration of Carbon Nanotube Network on Microelectromechanical Switch for Ultralong Contact Lifetime Eunhwan Jo, Min-Ho Seo, Soonjae Pyo, Seung-Deok Ko, DaeSung Kwon, Jungwook Choi, Jun-Bo Yoon, and Jongbaeg Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02747 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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ACS Applied Materials & Interfaces

Integration of Carbon Nanotube Network on Microelectromechanical Switch for Ultralong Contact Lifetime Eunhwan Jo†, ‖, Min-Ho Seo‡, §, ‖, Soonjae Pyo†, Seung-Deok Ko○, Dae-Sung Kwon†, Jungwook Choi#, Jun-Bo Yoon‡* and Jongbaeg Kim†* †

School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea

‡

School of Electrical Engineering, Korea Advanced Institute of Science Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea §

Information & Electronics Research Institute, Korea Advanced Institute of Science and

Technology (KAIST), 291, Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ○

#

Broadcom Ltd., 1730 Fox Dr, San Jose, CA 95131, USA

School of Mechanical Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea

‖These

authors contributed equally

KEYWORDS: carbon nanotube, switch, reliability, microelectromechanical system

ACS Paragon Plus Environment

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ABSTRACT

Micro-/nanoelectromechanical (MEM/NEM) switches have been extensively studied to address the limitations of transistors, such as the increased standby power consumption, and performance dependence on temperature and radiation. However, their lifetimes are limited owing to the degradation of the contact surfaces. Even though several materials and structural designs have been recently developed to improve the lifetime, the production of a microswitch that is compatible with complementary metal-oxide-semiconductor (CMOS) with a long lifetime remains a significant challenge. We demonstrates a vertically actuated MEM switch with extremely high reliability by integrating a carbon nanotube (CNT) network on a gold electrode as the contact material using a low-temperature, CMOS-compatible solution process. In addition to its outstanding mechanical and electrical properties of CNT, its deformability dramatically increases the effective contact area of the switch, thus resulting in the extension of the lifetime. The CNT-coated MEM switch exhibits a lifetime that is more than 7 × 108 cycles when operated in hot-switching conditions, which is 1.9 × 104 times longer than that of a control device without CNTs. The switch also shows an excellent switching performance, including a low-electrical resistance, high on/off ratio, and an extremely small off-state current.

1. Introduction Micro-/nanoelectromechanical (MEM/NEM) switches have attracted considerable interest as promising candidates for use in future integrated circuits to overcome the limitations of complementary metal-oxide-semiconductor (CMOS) transistors.1 Compared to conventional CMOS transistors that are associated with an unavoidable leakage current and a limited


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subthreshold swing, MEM/NEM switches have an outstanding performance, such as a near-zero leakage current, low-power consumption, and high on/off ratios.2-8 Moreover, unlike conventional solid-state devices,9 they show minimum performance degradation even under harsh environments, including high temperatures, external electric fields, or in the presence of radiation, which makes it suitable for use in diverse applications ranging from the automobile to the aerospace industries.10-12 However, the MEM/NEM switch generally suffers from limited lifetime because repeated physical contact would cause surface degradation and damages, including permanent stiction, material transfer, wear, welding, and oxidation.13-15 For example, many switches use gold (Au) as the contact material because of its excellent electrical and chemical properties, such as high conductivity, low-contact resistance, high-chemical resistance, and high-oxidation resistance. However, the low-elastic modulus, low-melting point, and high ductility of Au lead to surface damages in the case of repeated contact loading, thus limiting the stable and repetitive operation of the switches.16 Recently, a broad range of materials and structural designs of the switches have been proposed to improve their lifetimes.17-19 For example, the switches based on metal alloys, ruthenium, or silicon carbide have shown improved contact lifetimes.20-23 Moreover, there have been studies about all metal based NEM switches to improve the lifetime with a reduced contact resistance.24-25 For example, Y. Qian et al. reported studies about NEM switches fabricated by molybdenum (Mo) as contact material and structure by a single lithography step, and thus achieved a decreased contact resistance of several kohms even at the nanoscale contact area, which also showed repeatable operations at high temperature. However, the device performances were limited owing to their high-electrical contact resistances, which required increased bias voltages. The operation environment was also constrained only under vacuum conditions owing to their low-oxidation and chemical resistance.


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Emerging one-dimensional (1D) and two-dimensional (2D) nanomaterials, including nanowires, carbon allotropes, and transition metal dichalcogenides, have been explored to improve device performance, or to actualize nanoscale devices. Among the various nanomaterials, 1D carbon nanotubes (CNTs) and 2D graphene have attracted significant attention owing to their excellent mechanical and electrical properties, such as the exceptionally high Young's modulus26-27 and tensile strength,28 high-thermal,29-30 and electrical conductivities, as well as the advances in scalable manufacturing. In recent years, these carbon nanomaterials were used as highly reliable contact materials. For example, various carbon-based contacts, such as suspended CNT–diamond-like carbon contact,31 vertically aligned carbon nanotubes (VACNTs)–VACNTs contact, graphene-graphene contact,32 and graphite-graphite contact,33 have been previously developed for displacement sensing,34 dry adhesives,35 inertial microswitches,36 and electromechanical contact devices. NEM relays were also reported where suspended single-stranded CNT or CNT bundles contact conductive solid-state electrodes.10, 37-41 However, these studies demonstrated high-electrical resistance and low-contact durability owing to the permanent adhesion or burn-out during the switching operation. To address the low durability issues, Jang et al. introduced that ultrathin oxide coated electrode minimized damages of a nano beam owing to electrical discharges.8 O. Loh et al. also proposed durable NEM contact switches using a CNT and a conductive diamond-like carbon as a movable element and a fixed contact electrode to reduce stiction and improve reliability, respectively.31,

42

A microcontact

device was also developed by directly growing VACNTs on the sidewalls of microstructures.43 Despite its excellent contact durability at high-current densities, high processing temperature was required to synthesize VACNTs, and the applications of these devices were limited owing to their high-contact resistance (285 Ω), slow switching speed (184 µs), and large footprints (> 1


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mm2). In this work, we demonstrate a vertically actuated MEM switch with extremely high reliability using a solution-based CNT network integration process. Based on the reliable contact between randomly dispersed CNTs on the lower and top Au electrodes, our MEM switch exhibits low-contact resistance, near-zero leakage current, and a contact lifetime of up to 7 × 108 cycles during hot-switching operations in the presence of nitrogen (N2) at atmospheric pressure.

2. Result and Discussion 2.1 Switch fabrication and operation concept The proposed three-terminal switch consists of a CNT-integrated source electrode, a movable plate (drain) suspended by four crab-leg beams, and a fixed gate electrode. Figure 1a shows a schematic view of the lower fixed electrodes and the cross-section of the movable drain structure of the CNT–Au contact-based MEM switch. A schematic illustration for the fabrication process of the switch is shown in Figure 1b. In order to form the lower electrodes on a thermally oxidized silicon wafer, chrome (Cr) and Au (10 and 100 nm) were deposited and patterned by photolithography and evaporation (Figure 1b, (i)). CNTs were then coated on the entire wafer by immersing the wafer into a bath with CNTs dispersed, and patterned by selective etching using oxygen (O2) plasma (Figure 1b, (ii)). The detailed process used for integrating and patterning CNTs on the lower electrode is described in Supporting Information Figure S1. The upper movable electrode was fabricated by using two photoresist (PR) sacrificial layers and a nickel (Ni) electroplating process with a PR mold as follows. A gap between the lower electrode and the contact area of the upper electrode (gd) was set to 400 nm by the patterned first positive PR layer with a thickness of 400 nm (Figure 1b, (iii)). Following the hard baking of the first PR, a second positive PR layer with a thickness of 1.0 μm was coated on top of it and was


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subsequently patterned (Figure 1b, (iv)). A gap of 1.4 μm was formed between the gate electrode and the upper electrode (g0) through the first and second PR sacrificial layers. After depositing the Au layer (thickness of 100 nm) onto the entire wafer, a positive PR mold with a thickness of 8 μm was patterned for the electroplating process (Figure 1b, (v)). The Au layer was not only used as a seed layer for electroplating but also as a contact material on the opposite side of the CNTs. The upper movable electrode of the switch was then fabricated by electroplating Ni as the structural material. The switch was released by dissolving the PR layers and by using critical point drying. Additionally, the Au was also removed by a commercial Au etchant before the PR dissolution process, except the layer under the upper electrode (Figure 1b, (vi)). As a control, a device without CNTs (Au–Au contact) was also fabricated to investigate the effect of CNTs on the switching lifetime. The entire manufacturing process was performed on a 4-inch wafer and was compatible with CMOS technology because there was no involvement of any hightemperature process. The geometric parameters and dimensions of the fabricated switch are listed in Supporting Information Figure S2 and Table S1. Figure 1c shows a conceptual diagram of the behavior of the CNTs when the upper movable electrode approaches the lower electrode, as driven by the electrostatic force between the movable electrode and the static gate electrode at the bottom part of the switch. During the mechanical on/off operations, the CNT plays important roles not only to significantly reduce the physicochemical degradation at the contacting interface because of its mechanically and chemically stable properties, but also to cause the drastically lowered contact-resistance by generating many contacting sites. Figure 1d and e show the drain–source current (IDS)–bias voltage (VDS) characteristics of the switch at off- and on-states, which were measured by increasing the VDS from -100 mV to 100 mV with a step voltage of 5 mV. It should be noted that


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a linear IDS–VDS curve indicates the formation of an ohmic contact between the Au and CNTs even at the low actuation voltage (Figure 1d). When the actuation voltage increases, the contact force monotonically increases, and the movable electrode can thus press down more forcefully on the lower electrode. In this way, at higher voltages, the contact resistance (RC) is dramatically reduced to a few ohms owing to the increase of the actual contact area (Figure 1e). Scanning electron microscope (SEM) images of the fabricated switch are shown in Figure 2a and b. The movable electrode attached to a beam with a thickness of 7 μm is suspended above the fixed lower electrode and separated by a gap of 1.4 μm (Figure 2b). Figure 2c shows the switch array fabricated on a 4-inch wafer. To investigate the morphology of the CNT network on the lower contact electrode underneath the movable electrode, it was forcefully removed by breaking the Ni beam with a probe. Figure 2d and e show the optical and SEM images of the lower electrodes, respectively, demonstrating that the CNT network remains on the Au source electrode even after the completion of several wet processes, including PR spin coating, developing, rinsing, and electroplating. As can be observed in Figure 2f, the CNTs are clearly patterned by O2 plasma etching with a PR mask, and no significant damage to the Au layer is observed after the removal of the CNTs.

2.2 Characterization of switches Figure 3a shows the IDS with respect to the gate voltage (VG) at a VDS of 1.0 V and current compliance of 1.0 mA in air, indicating that the IDS dramatically increases (i.e., the electrical contact resistance decreases) at the threshold voltage. During the forward sweep of the gate voltage, the contact between the Au and CNT network occurred first at a VG of 67.4 V, and the current increased abruptly to a few μA from the unmeasurable off-state current (< 10 pA). As VG


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increased further, the designated contact area of the upper plate pressed the CNT network. Correspondingly, a greater number of CNTs were in contact.44 This would increase the actual contact area between the top Au surface and the CNTs at the bottom part, which would result in a decreased contact resistance. The IDS increased to 1 mA when the contact was fully formed at a VG of 71.8 V (VON), while the on-state resistance (RON) resistance dropped to 12.69 Ω. In the reverse sweep, the switch was completely turned off at a VG of 65.8 V (VOFF). The designed apparent contact area of the switch is 12 µm × 12 µm. The on/off ratio of our switch was higher than 108, as shown in the inset of Figure 3a. We also measured the dynamic switching response at 100 Hz under a square wave VG in the amplitude range of 0–70 V with a 50% duty ratio, while the VDS was maintained at 1 V (Figure 3b). The voltage drop across the external resistor (1 kΩ) was measured simultaneously with the VG (see Supporting Information Figure S3 for details of the experimental setup). The fact that the output voltage is distortion-free implies that the CNTs do not interfere with the high-speed switching operations of the device. The transient switching response was also investigated, and it exhibited a delay time of 1.6 μs for both the switching-on and switching-off processes (Figure 3c). The control device exhibited the delay time of 2.6 and 2.0 μs for the switching-on and off, respectively. The difference of the delayed times between the switch with CNT and control device may be originated to the difference gap owing to a thickness of the CNTs. The mechanical effects of the CNT at the contact-interface were confirmed from the double swept IDS–VG characteristics of the devices. Figure 3d presents two different hysteretic behaviors for a CNT–Au contact and an Au–Au contact at a VDS of 1 V having a same apparent contact area. The switch with the CNTs started to make contact at 67.4 V, was then completely turned on at 71.8 V in the forward sweep, and then turned off at 65.8 V in the reverse sweep. On the other


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hand, the control device (Au–Au contact) was turned on at 69.3 V and then turned off at 16.9 V showing a large hysteretic behavior. A large difference in VOFF between the two devices may originate from the difference in the adhesion forces (Fadhesion) of each device. In case of the proposed devices, the movable distance gd of the upper electrode was designed to be smaller than g0/3 to prevent pull-in during the operation of the switch, thus, the electrostatic force between the gate electrode and the plate is equal to the restoring force of the beam’s spring until the contact is formed.45 Therefore, Fadhesion of the switch is calculated as the difference of the electrostatic forces at VON and VOFF as an equation (Equation (1)),

Fadhesion = FON, electrostatic – FOFF, electrostatic =

 r 0 Agate

2  g0  gd

V 

2 ON

2  VOFF 

(1) (2)

where FON, electrostatic is the electrostatic force where the contact of the switch is formed, FOFF, electrostatic

is the electrostatic force at the instant of the separation of the contact, εr is the relative

dielectric constant of air, ε0 is the vacuum permittivity, Agate is the area of the gate electrodes, and Vcontact is the turn-on voltage of the Au–Au contact switch. In order to compare the two values of Fadhesion for the same apparent contact area, we assumed that the VON of the switch with the CNTs (CNT–Au) was equal to the VON of the control device (Au–Au), based on the designed movable distance of 400 nm (gd). As a result, the Fadhesion of the switch with CNTs (CNTs–Au) was calculated as 69.92 μN, while the switch without the CNTs (Au–Au) generates a Fadhesion with a magnitude of 667.87 μN (Equation (2)). The calculated Fadhesion of the switch with CNTs was approximately 10 times lower than that of the control device (Au–Au), thus implying that the


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failure that occurred owing to the irreversible wear30 and stiction during the operation can be reduced by employing CNTs. The electrical effects of the CNT at the contact-interface are also verified. The electrical investigation was performed by analyzing the electrical resistance with respect to the applied contact force by VG-overdriven operation at the on-state. Figure 3e shows the calculated contact force by the VG versus the resistances of the switch with CNTs and control device without CNTs (Au-Au contact) derived from the IDS–VG sweep curve after the contact of the switch was established. The drain-source voltage (VDS) of 10 mV is applied without a current compliance to measure the resistance of the switch with CNTs and the control device. Whereas the control device without CNTs has a low on-resistance of 12.18 Ω without the significant change of the resistance corresponding to the increased contact force, the switch with CNTs shows a drastic resistance-variation by the applied force. Once the device became the ‘on-state,’ the electrical resistance was very high (~10 MΩ), but it is drastically decreased by 49.61 Ω when all apparent contact areas of the upper electrode are involved in the contact. The resistance of the switch is then further decreased as the applied force is increased. We understand this result from the additionally generated contacting sites. As we pre-mentioned in Figure 1c, owing to the VGoverdriving operation, the higher contact force is induced and it generates the new contacting sites by deforming the CNTs. Supporting Information Figure S4 shows the contact forces versus the resistance curves derived from the IDS-VG curves for 10 cycles, showing reliable contact behaviors to repetitive contacts. We further measured the IDS–VG characteristics of the CNT device during the repetitive operations (VDS=0.1 V). During the 5-operation, the device with CNTs shows the repetitive on-resistance decrease by the VG overdriving, and, specifically, the device successfully generates the high-voltage contact with 10% of VG overdriving (Figure 3f). It


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should be noted that the IDS-VG characteristics at the low-voltage contact are not well matched during every cycle, but these results originated from the randomly dispersed CNTs configuration, as shown in Figure 2e, and the CNTs still play a role generating the new contact-sites every time.

2.3 Long-term lifetime measurements To verify the reliability and current-carrying capability of the CNTs as a contact material, we measured the lifetimes of the switches with and without CNTs. The electrical measurements were performed during operation in static applied voltage between the drain and source, defined as hot-switching conditions, with the bias VDS of 0.1 V, 0.1 V, and 1.0 V and the current compliance of 10 μA, 0.1 mA, and 1.0 mA in air, respectively. The lifetimes of the switches were observed at 300 Hz with the use of square wave-VG in the range of 0–90 V with a 50% duty ratio, until the on-state IDS reached the limited current. The input current was controlled by changing the resistance values of the resistors of the test equipment (Keithley 2602 Sourcemeter) while the applied bias voltage was maintained, and thus the value of the maximum IDS was limited by the set value.33,

46-47

The value of the on-state IDS in air shown in Figure 4a was

measured every 1 × 106 cycles from 1 × 106 cycles to failure. The lifetimes of the switches without CNTs (Au–Au) were significantly shorter. Because several of these had life cycles that were smaller than a few hundred, the test was performed with significantly lower frequency inputs (1–10 Hz) to observe their short lifetimes. As shown in Figure 4a, the switch with CNTs operated for over 11 million cycles at the IDS of 10 μA, and for over 3 million cycles at the IDS of 0.1 mA. Even for an IDS value of 1.0 mA, the switch yielded a lifetime that exceeded 1 million cycles with the current density of 694.4 A/cm2 (12 μm × 12 μm contact area). By contrast, the control device (Au–Au contact) operated for 1.9 × 105 and 2 × 104 cycles at the IDS values of 10


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μA and 0.1 mA, respectively. At a carrying current of 1.0 mA, the control device failed after several hundred cycles owing to an electrical short. The results indicate that the switch with CNTs exhibited a lifetime that was 57 times longer than that without CNTs at the IDS of 10 μA. This difference in lifetimes increased as the carrying current increased. Specifically, the difference was 150 times at 0.1 mA, and ~3,000 times at 1.0 mA, which was due to the different failure modes of the switches with and without CNTs. All measured switches with CNTs failed owing to the increased contact resistance caused by the oxidation of CNTs. No electrical migration or welding was caused by Joule heating since the current was distributed over the increased effective contact area with the deformability of the CNTs. However, the failure mode of the control device (Au–Au) changed when the IDS increased from 10 μA to 1.0 mA. The failure of the control device (Au–Au) was due to the abrupt increase of the contact resistance that was attributed to wear at low currents (10 μA and 0.1 mA). At the IDS of 1.0 mA, the increased current density resulted in the melting of the Au surface, and the lifetime decreased abruptly. The relationships among the lifetime, failure mode, and the carrying current of the control device (Au–Au) are consistent with a previously published study based on the Au–Au contact.14 The low adhesion and deformability of the CNTs were also investigated during the long-term operation with a carrying current of 10 μA (Figure 4b). Figure 4b shows the measured IDS–VG sweep curves after 1, 102, 104, and 106 cycles, respectively. At a typical conductive solid–solid contact, the contact resistance changed only slightly owing to the elastic deformation induced by the contact force after the contact.15 On the contact interface of the CNTs–Au, the increased effective contact area that resulted from the deformability of the CNTs dramatically reduced contact resistance. An abrupt decrease of the resistance was observed on all measured IDS–VG curves without significant hysteresis, which implied that the deformability of the CNTs was


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maintained even after repeated contacts (> 1 million cycles) in hot-switching condition at the IDS of 10 μA and the VDS of 0.1 V. The VON and the VOFF decreased from 76.5 V to 72.5 V and from 65 V to 61.5 V during the span of 106 cycles of operation, respectively. As can be observed in Figure 4b, the changes in the VON and VOFF would be caused by the decrease of go, which possibly originates from the plastic deformation of the beam by repeated loading. The decreased go is also observed as shown in Supporting Information Figure S5 by using a scanning white light interferometry microscope. In order to verify the reproducibility of the switch, we prepared nine switches and three experimental conditions (IDS of 10 μA, 0.1 mA, and 1.0 mA in air). For each condition, three switches were tested simultaneously, and the results are presented in Supporting Information Figure S6. For all of the switches, the measured lifetime exceeded 106 cycles in hot-switching conditions with different values of the IDS. In addition, there was no irreversible stiction caused by adhesion or welding until the switch failed owing to the increased resistance, followed by the termination of the measurements. To examine the contact surface after failure, we forcefully detached the upper movable electrode and observed the contact surface of the upper and lower electrodes. In Supporting Information Figure S7, the SEM and atomic force microscope (AFM) images show the contact surface of the upper electrode after 30 million cycles of operation with a current of 1.0 μA and a VDS of 0.01 V (Figure S7a and S7b),48 and 8 million cycles of operation with a current of 1.0 mA and a VDS of 1.0 V (Figure S7c and S7d) in hot-switching conditions in air. Supporting Information Figure S8 also shows the AFM image and the profiles of the lower contact electrode after the failure at 1.0 mA. Unlike a typical Au–Au contact, there was no material transfer, melting, electromigration, or vaporization of Au. However, few CNTs were transferred from the


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lower fixed electrode to the upper contact area as can be seen in the SEM and the AFM images (Figure S7 and S8). While there were several different failure modes for typical MEM switches, such as permanent stiction, welding, melting, material transfer, and increased resistance, the only failure mode of our CNT-integrated switch was the increased contact resistance. The increased resistance at the contact interface of the CNT–Au can be attributed to the oxidation and decomposition of CNTs owing to Joule heating in air.49 In order to know the decomposition behavior of CNTs, we prepared six identical switches and measured the Raman spectra of the CNT network in the lower contact area after 1 × 105, 2 × 105, 3 × 105, 5 × 105, 6 × 105, and 2 × 106 cycles at the IDS of 1.0 mA and the VDS of 0.1 V as shown in Supporting Information Figure S9, respectively. During the switches operated without failures, there was no significant change on the D and G band which exhibited the existence of multi-wall CNTs. However, the D and G band were significantly decreased at a device after 2 × 106 cycles. The result is consistent with previous researches that showed the decrease of the D and G band by the destruction behavior of CNTs.5052

To explore the relationship between the oxidation of CNTs and the lifetime of the switch, and to confirm the effect of the N2-encapsulated packaging, we measured the long-term lifetime in dry N2 at atmospheric pressure with the bias VDS of 0.1 V and 1.0 V and the current compliance of 0.1 mA and 1.0 mA, respectively. The lifetimes were observed at 1000 Hz for a 50% duty cycle for a square wave VG in the range of 0–90 V. The value of the on-state IDS in N2 shown in Figure 4c was measured every 1 × 106 cycles for 2 × 107 cycles, every 1 × 107 cycles for 1 × 108 cycles, and 1 × 108 cycles to failure, respectively. As shown in Figure 4c, the switch with CNTs exhibited a lifetime that was more than 15 times longer (18 million cycles) than that in air at 1.0


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mA in the presence of N2. The switch with CNTs also exhibited a lifetime of 700 million cycles in N2 at 0.1 mA, which was 1.9 × 104 times longer than that of the switch without CNTs in N2, 3.5 × 104 times without CNTs in air, and 70 times with CNTs in air, respectively. However, the control switch (Au–Au contact) operated at approximately 1.5 × 103 cycles at 1.0 mA, and at approximately 3.7 × 104 cycles at 0.1 mA. Since Au is a noble metal with a high oxidation resistance, no considerable differences in the contact lifetimes between air and N2 were observed in the control devices. The failure of the switch with CNTs in N2 was caused by the irreversible welding after the extinction of CNTs in all the devices, and no rapid resistance increase was observed in various conditions in all tested switches with CNTs. To confirm that this failure mode of welding originated from the extinction of CNTs and the change of the contact surfaces, we separated and inspected the upper and lower electrodes. Figure 4d and 4e show SEM images of the contact surfaces on the upper and lower electrodes, respectively, where welding occurred after 700 million contacts at 0.1 mA in the presence of N2. No CNTs were observed on the contact surfaces of the upper and lower electrodes. The transferred Au from the lower fixed electrode are found in the SEM image of the upper electrode (Figure 4d). On the lower electrode (Figure 4e), melting and detachment of Au are observed at the contact surface. Figure 4f shows the Raman spectra of the electrodes. Contrary to the noncontact area, the D and G bands of the CNTs are not shown on the contact areas of either the upper or the lower electrodes. It is noted that CNTs could be decomposed by Joule heating with repetitive contact in hot-switching conditions. Figure 4g shows the magnified SEM image of the boundary of the contact area in the lower electrode. Once the CNTs are removed, that is, a direct Au–Au contact is established, the welding occurs at the contact surface.


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3. Conclusion We developed a vertically-operated MEM contact switch with a significantly enhanced lifecycle by integrating a CNT network on Au electrodes using a simple, low-temperature, and solution-based CNT deposition process. The experimentally demonstrated lifetime was over 700 million cycles in hot-switching conditions, and an excellent switching performance was achieved, that included a low on-state resistance, high on/off ratio, immeasurable off-state current, and a high-current capability. Contrary to typical solid–solid contact devices, by integrating CNTs onto the contact surface, we can prevent the contact surface problems, such as stiction, welding, and material transfer, thus enabling long-term operation with low adhesion at the contact interface between the CNT network and the solid. Experimental results revealed that the switch with CNTs has a lifetime that is several tens to several thousand times longer than that of the control device without CNTs. The oxidation effect on the lifetime and the effectiveness of N2 packaging of the CNT network-based switch were confirmed by the cyclic switching test given that the lifetime before failure was extended up to 70 times compared to that in air. Based the extended lifetime and the scalable solution-based fabrication process, we expect that the CNT network can be applied to diverse nano-, micro- or macro scale contact devices, to extend their lifetimes and enhance their contact reliabilities.


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4. Experimental Section Fabrication of Carbon Nanotubes Solution: 2 wt% multi-wall CNT (MWCNT) (diameter: 20 nm) ink was purchased from Applied Carbon Nano Inc. and dispersed in N,NDimethylmethanamide solution (2 mg/ml) via sonication for 6 h. Carbon Nanotubes Coating: To improve the adhesion capability of the Au electrode, the wafer was immersed in poly-L-lysine solution (Sigma-Aldrich) for 9 min following O2 plasma treatment for 90 s with 100 W and 20 sccm (COVANCE-MP, Femto Science). After washing with deionized (DI) water, the wafer was immersed in a CNTs-dispersed bath for 12 h. The wafer is washed with DI water and dried with an N2 gun. Then the CNT-coated wafer is baked at 110ºC for 90 s to remove the solvent. The average thickness, length, and number of CNTs per unit area (µm2) were estimated as 23.38 nm, 0.34 µm, and 33.7/µm2, respectively, which were measured and counted at randomly selected three regions by SEM and AFM. Carbon Nanotubes Removal and Patterning: To pattern the CNTs, a negative PR (DNR-L300, Dongjin Semichem Inc.) was used for a mask. The CNTs was removed by using reactive ion etching with a power of 100 W and O2 of 20 sccm flow rate for 40 s. Then the PR mask was dissolved by using acetone with sonication. Measurement of Transient Response: A Function generator (33220A, Agilent) and a voltage amplifier (F20AD, FLC Electronics) were used to apply a square wave. The bias voltage was applied through a d.c. power supply (E36477A, Agilent). An oscilloscope (DSO5014A, Agilent) was used to simultaneously record and monitor the voltage drop across the resistor of 1 kΩ and the input of the square wave from the function generator. Three-dimensional Profile Measurement: To measure the surface profile without contact, a scanning white light interferometry microscope (NewView 6000, Zygo Corporation) was used.


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FIGURES

Figure 1. (a) Schematic of lower fixed electrodes and cross-sectional view of an upper movable drain structure of carbon nanotubes (CNTs)–gold (Au) contact-based switch. (b) Schematic diagram of the fabrication process of the switch. The source electrode contacts a square contact


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area of an Au layer under the movable drain structure. Control devices without CNTs (Au–Au contact) were also fabricated to investigate the effect of CNTs on the switching lifetime. (c) Conceptual diagram of the behavior of the CNTs when the upper movable electrode approaches the lower electrode owing to the electrostatic force between the upper movable electrode and the lower static gate electrode. The circles (blue, red) indicate the contacting spots of the proposed switch. (d) and (e) Measured drain–source current (IDS)–bias voltage (VDS) curves indicating the formation of an ohmic contact between CNTs and Au.


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Figure 2. Images of the fabricated switches. (a) SEM image of the fabricated switch. (b) SEM image showing the movable electrode attached to a beam with a thickness of 7 μm, suspended above the lower fixed electrode with a 1.4 μm gap after release. (c) Optical image of the switch array fabricated on a 4-inch wafer. (d) Optical image of the lower fixed electrodes after the movable electrode was forcefully removed by breaking the Ni beam with a probe. (e) SEM image of the CNT network. The CNTs on the Au source electrode remain even after the completion of several wet processes. (f) SEM image of the boundary between the patterned CNTs and the Au electrode under the CNTs after the completion of the O2 plasma etching.


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Figure 3. Characterization of the switches. (a) Measured drain-source (IDS) as a function of the corresponding applied gate voltage (VG) of the switch with the use of CNTs. The inset shows that the on/off ratio of our switch is larger than 108 and the decreased contact resistance was attributed to the deformability of the CNTs. (b) Dynamic response of the switch with CNTs during operation at 100 Hz. The output voltage without a distortion demonstrates that the CNTs do not interfere with high-speed switching operations. (c) Transient switching response when the device is switching on and off. The switch exhibits a delay time of 1.6 μs for both the turn-on and turn-off states. (d) Two different hysteretic behaviors of the CNT–Au contact and the Au– Au contact (control device). The large difference in VOFF between two devices originates from the difference in the adhesion force of each device. (e) Calculated contact forces versus the resistance curves of the switch with and without CNTs. (f) Measured IDS-VG sweep curves for 5 cycles exhibiting a reliable and small hysteretic contact behavior.


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Figure 4. Long-term lifetime measurements of the switches with and without CNTs in air and N2. (a) Lifetime measurement of the switches was performed in hot-switching conditions with the bias VDS of 0.1 V, 0.1 V, and 1.0 V and the current compliance of 10 μA, 0.1 mA, and 1.0 mA in air, respectively. The failure with permanent stiction was not observed among all measured switches with CNTs in air. (b) IDS–VG log-linear sweep curves measured after 1, 102, 104, and 106 cycles in hot-switching condition at the IDS of 10 μA with the VDS of 0.1 V. As observed, the deformability of the CNTs was still maintained during the span of 106 cycles of operation. (c) Long-term lifetime measurements in dry N2 at atmospheric pressure. The switches with CNTs were operated over 700 million cycles and 18 million cycles in hot-switching


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conditions with the bias VDS of 0.1 V and 1.0 V and the current compliance of 0.1 mA and 1.0 mA, respectively. Contrary to the failure of the switches operated in air, all switches with CNTs failed owing to the irreversible welding in the presence of N2. (d) and (e) SEM images of the contact surface of the upper and lower electrodes, respectively, in the welded switch after 700 million cycles at the IDS of 0.1 mA. The inset in (d) shows the tilted SEM image of the upper contact area. (f) Raman spectra of the contact area in the lower- (black open circle in (d)) and upper electrode (blue open circle in (e)) and noncontact area (red dotted-line circle in (e)) in the lower electrode. The D and G bands of MWCNTs can only be observed within the non-contact area. (g) Magnified SEM image of the boundary of the contact area in the lower electrode. No CNTs are observed within the contact area of the upper or lower electrode.


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Supporting Information Figure S1. Fabrication processes of carbon nanotube (CNT) coating and a surface treatments. (PDF) Figure S2. A scanning electron microscope (SEM) image of the switch with geometric parameters. (PDF) Table S1. Geometric parameters, symbols, and dimensions of the fabricated switch. (PDF) Figure S3. Experimental setup for the measurements of the transient response. (PDF) Figure S4. Calculated contact force versus the resistance curves for 10 cycles. (PDF) Figure S5. A non-contact surface three-dimensional profilers using a scanning white light interferometry microscope. (PDF) Figure S6. Measured lifetimes of the nine devices. (PDF) Figure S7. SEM and atomic force microscope (AFM) images of upper contact areas after failure. (PDF) Figure S8. AFM images and profiles of a lower contact area. (PDF) Figure S9. Raman spectra of a lower contact area for repeated contacts. (PDF)

Corresponding Authors *E-mail: [email protected] (J.K.). *E-mail: [email protected] (J.-B.Y).


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the Commercialization Promotion Agency for R&D Outcomes(COMPA) funded by the Ministry of Science and ICT(MSIT) [2018K000285] and by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. NRF-2018R1A2A1A05023070).

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ToC figure


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Integration of Carbon Nanotube Network on Microelectromechanical

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