RF NEM Capacitive Switch Based on Dense Horizontal Arrays of CNTs
D. Acquaviva", A. Arun", S. Esconjaureguib, J. Cao", R. SmajdaC, D. Buovet", A. MagrezC, L. Forroe, J. Robertsonb and A. M. Ionescu"
b aNanolab and cLNNME, EPFL, Lausanne, Switzerland, Engineering Department, Cambridge University, Cambridge, UK email:
[email protected], phone: +41 21 693 3971, fax: +41 21 693 3640
Carbon nanotubes (CNTs) have been considered one of the most promising material to make nanodevices, in particular nanoelectromechanical systems (NEMS), due to their remarkable mechanical (Young's modulus �1TPa) and electrical (high conductivity for metallic nanotubes) properties, nevertheless their low mass. Several NEMS device categories based on carbon nanotubes have been reported in the literature: sensors [1], non-volatile memories [2], tunable oscillators [3] and nanoswitches [4], which offer high speed at low voltage and power. However, the majority of these works are based on individual tubes, with poor control of the device fabrication, which makes questionable their interest for integrated circuit applications. Hayamizu et al [5] reported the fabrication of nanorelays based on dense arrays of nanotubes, starting from a CNT-wafer. Based on the CNTs array configuration, here we present, for the first time, the fabrication and the characterization of a capacitive shunt NEM switch whose movable electrode is made up of a horizontal CNTs membrane, that could be used for future RF applications. Figure l(a) reports the schematic cross-section of the RF NEM switch. The switch is loaded on a coplanar waveguide (CPW). The bottom electrode is constituted by a W line (200 om thick, IOflm wide) and AIN (100nm thick) is used as dielectric layer. The anchors for the suspended electrode are made of LTO. The two TiN (50 om) ground-plane conductor of the CPW are patterned on the top of the LTO layer; this metal choice is due to the fact that carbon nanotubes are able to grow directly on it. The poor conductibility of TiN is improved by depositing a Pt layer on top of it. Catalysts (Fe on top of Ab03) are patterned on TiN layer in order to have a selective CNT growth. The CNT synthesis has been performed by thermal CVD at atmospheric pressure. The growth temperature is 600°C and ethylene is used as carbon source. Due to the high aspect ratio of the catalyst dimensions, even if CNTs start growing vertically are pushed down to the substrate by the gas flow. This aligns CNTs horizontally in the direction of the flow (Fig. 2(a)). This results in high dense (1012/cm2) array of horizontal SWNTs 100 flm long. An isoprophanol treatment (5 minutes) was performed to densify the CNTs membranes in horizontal position. This treatment shrinks the membrane from 5flm to 500 om in thickness and from 50 to 40 flm in width (Fig. 2(b)). The last step of the fabrication, before the releasing, is the metallization of CNTs arrays: Pt contacts are patterned by E-beam lithography followed with releasing by HF vapour for 12 minutes, Fig. 2(c). The achieved air-gap is on the order of 500 nm. The devices have been characterized both at DC level and at high frequencies (RF). All measurements have been performed in vacuum and at room temperature and they are referred to the device with a CNT membrane 60 flm x 40 flm x 0.5 flm. Pull-in and pull-out values are confirmed by Fig. l(b) which shows the C-V curve of the MEMS capacitance measured with an impedance analyzer at high frequencies (1 GHz). The capacitance varies from 330 to 420 fF, with a tunability of 30%. These values include the parasitic capacitance of the CPW. Fig. l(c) shows the I-V (leakage) characteristic of the switch. The pull-in occurs at 6 V and the pull-out around 2 V. The leakage through the AIN insulator is low in all the experimental range, of the order of 40Wflm2 at 1OV. Based on the measured pull-in voltage of the fabricated membranes, we have extracted by using appropriate electromechanical models [6] a value of the equivalent Young's modulus of 8.5 GPa. Additionally, we investigated the equivalent electrical conductivity properties of the CNTs arrays by realizing a test 4-probe structure based on e-beam processed metal electrodes on top of the CNT array. Fig. 3(a) shows the I-V plot of the CNT membrane using the 4-probe method. The extracted equivalent material resistivity is in the order of 0.008 ncm, reflecting a mixture of both metallic and semiconductive SWNTs. High frequency measurements of the CNT-array capacitive switch have been performed with a HP 8753D (30 kHz-6 GHz) network analyzer. The S-parameters measurement of the capacitive NEM switch, in ground signal-ground (GSG) configuration, is presented in Fig. 3(b). The figure shows the isolation (S1 ) between port 1 2 and port 2 for the device up to 6GHz, both for the on and off state of the switch. In the up state, the isolation is of few dB; this is probably due to the roughness of the bottom electrode generated during the releasing. After the electrostatic actuation of the switch, the isolation is of 10 dB at high frequencies (5 GHz). The S-parameters have also been used to extract the lumped components of the device equivalent circuit whose schematic is depicted in Fig. 3(c). The variable capacitance ranges from 320 to 380 fF, always including the capacitance of the CPW. The extracted values are in the same order of magnitude of the ones reported in Fig. l(b). In conclusion, we have investigated the DC and RF properties of NEM capacitive switches based on horizontal dense arrays of CNTs, based on growth and surface micromachining processes that enable future wafer level control and integration of this family of devices. [1] [2] [3 ]
J. W. Ward, et al. Non-Vol. Mem. Tech.Symp., J. Kong, et al. Science 2000,287,622 V. Sazonova, et al. Nature 2004,431,284
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[4 ] [5] [6]
S. W. Lee, et al. Nano Lett.
2004,4,2027
Y. Hayamizu, et aI., Nature Nanotech. 2008,3,289 G. M. Rebeiz, RF MEM: Willey, 2003, Hoboken, NJ
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Figure 1: (a) Schematic cross-section of the CNT MEM switch in up and the down states. The cross-section schematic depicts the materials used for the device fabrication. (b) Experimental C-V curve of the CNT-array NEM switch, at lGHz, showing clear pull-in and the pull-out. The 330fF capacitance level in the suspended state is dominated by the pad contact capacitance. (c) Leakage characteristics of capacitive switch; pull-in and pull-out voltages of 6 V and 2 V, respectively. Following mechanical contact an increased leakage through the AIN insulator is obtained (however its value remains lower that 100pA for a contact area �60x40Jlm2).
Figure 2: SEM pictures of the fabricated device at three different steps: (a) after CNTs growth, (b) after isoprophanol (IPA) treatment, (c) after metal contact and releasing. The inset of each figure shows the CNT array thickness, the significantly decreasing of its thickness after the IPA treatment and the air gap of the CNT MEM switch, respectively.
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