Report on design of the CNT based RF switch

Transcription

1 CARBON BASED SMART SYSTEM FOR WIRELESS APPLICATION Start Date : 01/09/1 Project n Duration : 36 months Topic addressed : Very advanced nanoelectronic components: design, engineering, technology and manufacturability WORK PACKAGE : Design and simulation activities DELIVERABLE D. Report on design of the Due date : T0+1 Submission date : T0+13 Lead contractor for this deliverable : IMT Dissemination level : PU Public

2 /17 WORK PACKAGE : Design and simulation activities PARTNERS ORGANISATION APPROVAL Name Function Date Signature Prepared by: S.Xavier R&D Engineer 05/08/13 Prepared by: M.Dragoman Senior Researcher 1 Approved by: Afshin Ziaei Research Program Manager 05/08/13 DISTRIBUTION LIST QUANTITY ORGANIZATION NAMES 1 ex Thales Research and Technology TRT Afshin ZIAEI 1 ex Chalmers University of Technology CHALMERS Johan LIU 1 ex Foundation for Research & Technology - Hellas FORTH George KONSTANDINIS 1 ex Laboratoire d Architecture et d Analyse des Systèmes CNRS-LAAS George DELIGEORGIS 1 ex Université Pierre et Marie Curie UPMC Charlotte TRIPON- CANSELIET 1 ex National Research and Development Institute for Microtechnologies IMT Mircea DRAGOMAN 1 ex Graphene Industries GI Peter BLAKE 1 ex Thales Systèmes Aéroportés TSA Yves MANCUSO 1 ex SHT Smart High-Tech AB SHT Yifeng FU 1 ex Universita politecnica delle Marche UNIVPM Luca PIERANTONI 1 ex Linköping University LiU Rositsa YAKIMOVA 1 ex Fundacio Privada Institute Catala de Nanotecnologia ICN Clivia SOTOMAYOR 1 ex Tyndall-UCC Tyndall Mircea MODREANU

3 3/17 CHANGE RECORD SHEET REVISION LETTER DATE PAGE NUMBER DESCRIPTION Template 07/013 V1 06/09/ TRT Contribution 3

4 4/17 CONTENTS 1 INTRODUCTION 5 SIMULATION OF NEMS SWITCHES 7.1 THEROTICAL INTERPRATATION OF THE NEMS MECHANICAL SWITCH 7. SIMULATION OF ELEMENTARY NEMS MECHANICAL SWITCH Variation of the Young s Modulus, E 11.. Variation of the contact length between CNTs, L cont 1..3 Variation of diameter of CNTs and distance between CNTs 13 3 RF NEMS DESIGN AND SIMULATION RF NEMS DESIGN RF NEMS SIMULATION 16 4 CONCLUSION 17 4

5 5/17 1 INTRODUCTION The NEMS (nanomechanical systems) will replace soon in certain applications the MEMS (micromechanical systems) do to the fact that NEMS are smaller, faster and consume less energy than MEMS. NEMS are using new nanomaterials as carbon nanotubes (CNTs) or graphene. We will present in this task the design of a microwave switch based on CNT. The performances are better compared to MEMS regarding switching time and actuation voltage. A nanoelectromechanical system (NEMS) is a simple mechanical system such as cantilever, tweezer, double clamped beam, which is electrostatically actuated and having at least one dimension of few nanometres. NEMS could be made from various materials such as Si, SiC, GaAs, Au, metallic carbon nanotubes (CNTs) or metallic nanowires (K.L. Ekinci and M.L. Roukes, Nanoelectromechanical systems, Rev. Sci. Instr.76, (005). NEMS have mechanical resonance frequencies in the range 100 MHz-5 GHz, but could attain even 0 THz (Z. Isipov, D. Wolf, and A. Hassanein, Nano Letters 6, 1843 (006).so coinciding with the microwave, millimeterwave and sub-millimeterwave electromagnetic spectrum. NEMS have very low masses and very high mechanical quality factors attaining Nanorelays with switching time of few ns and tunable resonators based on NEMS are recently reported (S. Wook, D.S. Lee, R.E. Morjan, S.H. Jhang, M. Sveningsson, O.A. Nerushev, Y.W. Park, and E.E.B. Campbell,Nano Letters 4, 07 (004).,K. Jensen, C. Girit, W. Mickelson, and A. Zettl, Phys. Rev. Lett. 96, 15503(006),H.B. Peng, C.W. Chang, S.Aloni, T.D. Yuzvinsky and A. Zettl, Phys. Rev. Lett. 97, (006).(but there are still very few devices, which are exploiting the coupling between GHz mechanical oscillations of NEMS and microwave signals working in the same spectral frequency range. At a larger scale termed as microscale, there are a lot of applications based on MEMS (micro electromechanical systems) and microwave devices. MEMS are also basic mechanical configurations like cantilevers or double-clamped beams but much bigger than NEMS; when these MEMS are integrated with microwave devices or circuits, the entire circuit is termed as RF MEMS. These switches have widespread applications in the millimeter waves range due to their lower losses and higher isolation than switches based on semiconductor devices such as MESFET or PHEMT transistors or PIN diodes. However, the switching times of RF MEMS are in the range of -0µs, which is very slow in comparison with those displayed by semiconductor switches, which are in the range 0.-1 ns so they are not able to work for high-speed applications. When NEMS are integrated with microwave planar waveguides, such as coplanar waveguide (CPW), the resulted NEMS microwave switches display higher performances than their microscale counterparts i.e. RF MEMS switches. In this way, the low losses and high isolation gained via RF MEMS switches is preserved but the switching time is drastically reduced up to tens of ns or lower. Such high speed devices are a must for future communications systems for space and automotive applications. Therefore, NEMS target the most advanced communication or computing applications. This is demonstrated by the recent discovery of the nanotube radio, which consists of a CNT cantilever that emits electrons via the field-emission effect, the emission being modulated by a radio station signal and further detected by a cathode located in vacuum near the vibrating CNT resonator. In principle, a single vibrating CNT resonator performs the basic functions of a rudimentary radio able to sense the signal of a single radio station (K. Jensen, J. Weldon, H. Garcia, and A. Zettl, Nanotube Radio, Nano Lett. 7, , 007.This CNT radio, although still not able of tuning for sensing various radio stations, is 4 5 orders of magnitude smaller than the present radios implemented with the most advanced semiconducting technologies, and thus is targeting biological applications at subcellar level. 5

6 6/17 Very recently( J.O lee et al., Nature Nanotechnology 8, pp , 013) it is reported a NEMS switch Having actuation of 1V This result is due to the incorporation of a 4-nm-thick air gap, which is the smallest reported so far for a NEM switch generated using a top-down approach. However, this NEMS is not easily reproducible due to the fact that this NEMS attains the limits of clean room technologies. Below, there is a table with the main configurations and properties. of NEMS (Owen Y. Loh and Horacio D. Espinosa, Nanoelectromechanical contact switches, Nature Nanotechnology 1, pp ) being a condensed SoA of NEMS contact switches, all of being NEMS switches in various configurations. Due to the shortcuts with when two metallic NEMS are contacted the RF NEMS CNT switch presented below is avoiding such serious drawback by an initial polarization of CNTs which are switching with another pair of CNTs. Tableau 1 : Representative NEM-switch architecture 6

7 7/17 SIMULATION OF NEMS SWITCHES.1 THEROTICAL INTERPRATATION OF THE NEMS MECHANICAL SWITCH TRT has investigated the realization of an elementary NEMS switch in order to study the technological feasibility and the electrical/mechanical issues. Based on the concept developed by Jang et al. ( Nanoelectromechanical switches with vertically aligned carbon nanotubes APL Vol. 87, Issue 16, 005) [1], a tweezer configuration for an ohmic switch has been identified for preliminary device fabrication. The clear objective of this study is to demonstrate the activation of the CNTs under actuation. Figure 1 : NEMS switch, tweezers configuration Figure 1 above illustrates a typical NEMS switch design in the tweezer configuration as described by Jang et al []. In this particular configuration, a voltage is applied between a pair of CNTs and the commutation of the CNTs is obtained by a combination of attractive and repulsive forces. This design also allows for the CNTs to be released upon reset. This type of geometry has been studied for an ohmic contact configuration. The equation described below has been taken into account for the simulation purposes. The movement of a CNT in a NEMS switch can be obtained by the Equation 1 below [3] : 4 d W Equation 1 : EI = Felect + FvdW + FC + Felast + P 4 dx where W corresponds to the movement of the CNT, E is the Young s Modulus, I the inertia moment, F elec the electrostatic force applied for actuation, F vdw is the Van der Waals force corresponding to the force of attraction of atoms at low electrical intensity, F c is the Casimir force corresponding to the attraction force between parallel structures close to each other, F elast is the elastic force causing the movement of the CNT and P is weight of a CNT It must be noted that considering the weight of a CNT per unit length is of the order of Kg/m and that the geometry of a CNT is not in one plane only, it can be assumed that the weight and the Casimir force will be negligible compared to the other forces. The electrostatic force, F elec, can be derived from the electrostatic energy for a switch as given in Equation and Equation 3 [4,5] 7

8 8/17 Equation : E 1 elect = C( r) V with C ( r ) Equation 3 : L F L elect d = ( E L) elect dr = r ( r + R) = ln r R πε V + ε π 0 r R + 1 ( r R) r r + ln 1+ + R R where V is the actuation voltage, ε 0 is the vacuum permittivity, L, length of the CNT, C(r) the coupling capacitance between the CNTs, R the external diameter of the CNT and r is the distance between the CNTs. 1 For a tweezer configuration, the electrostatic force, F elas, is given by Equation 4 [6] Equation 4 : 8EI r L F elast 3 = with π D I = 4 ext 4 D where E is the Young s Modulus, I the inertia moment, L, length of the CNT and r is the distance between the CNTs. int 4 The Van der Waals forces, F vdw, can be summarised in Equation 5 considering the fact that the repulsive component of F vdw can be neglected and that the force is negligible when the CNTs are apart because the distance apart is too long. Equation 5 : EvdW d 1 L C R F π 6ρ ρ vdw = = L 5 dr 16r where E is the Young s Modulus, I the inertia moment, L, length of the CNT and r is the distance between the CNTs. C In order to determine the geometry of the design, it is fundamental to determine the criteria of reversibility. For this tweezer configuration, reversibility is achieved if the release force (the elastic force of the CNTs) is greater than maintaining force (the Van der Waals force) and is symbolized by the ration : (F elas /F vdw ). This ratio has to be > 1 for the elastic force of the CNTs to compensate the Van der Waals force. This enables the CNTs to be connected at actuation and released after the electrostatic force is removed. The actuation of a NEMS mechanical switch based on MWCNTs is exclusively related to the relationship between the elastic force of the MWCNTs ( F elas ) and the electrostatic force induced by the applied actuation voltage ( F elec ) assuming that F vdw in negligible upon actuation. In order to determine the minimum actuation voltage, V, the equation between the elastic force and the applied electrostatic force needs to be resolved as shown in Equation 6 and Equation 7 below. F elast + F elect Equation 6 : 0 = 8

9 9/17 Equation 7 : V = 8EI 3 r r 3 L 5 3 r 3 3 r r R ln R 3 3 r r + R R Lπε 0 where E is the Young s Modulus of the MWCNTs, I is the inertia, L, the length, R, the diameter, r the initial distance separating the MWCNTs and ε 0 =1/(36π.10 9 ) x10-1 F/m, the vacuum permittivity. The deflection of the MWCNTs can be expressed as in Figure below. The deflection of (3/5).r corresponds to the point of instability beyond which the applied tension is sufficient for actuation of the MWCNTs to occur. Figure : CNT deflection on applied voltage The actuation and reset of the NEMS switch obey a hysteresis cycle loop as shown in Figure 3. Figure 3(a) shows the variation of the deflection against the applied voltage to the MWCNTs and the point of instability. Figure 3(b) shows the activation and reset cycle. (a) (b) Figure 3 : deflection of the CNT vs applied voltage It is hence possible to determine the voltage at which the Van der Waals force (F vdw ) and the electrostatic force will no longer compensate for the elastic force for a defined geometry. Equation 8 : 9

10 10/17 V reset = 16EI ( r σ 3 σ ) ( σ F R) ( R) vdw L L ln ( 1 σ σ σ C ) ( L L C ) 3 πε L where σ is the equilibirum distance of Van der Waals between carbon atoms (0,34nm) and L C is the contact length between MWCNTs ( 116nm). Based on this summarized theoretical explanation, a novel design has been chosen for this proof-ofconcept validation based on a tweezer configuration with 4 electrodes, compared to 3 electrodes as described by Jang et al.. The design is shown in Figure 4. 0 R C R L CNT E : Young modulus Φ: diameter d contact Ti(10nm) / Mo (100nm) SiO (500nm) Silicon (300µm) R= 100nm 300nm V polarization V Actuation Figure 4 : 4 electrodes concept for tweezer configuration This design consists of two pairs of electrodes : a voltage V 1 will be applied to the inner pair of electrodes responsible for the connection of the MWCNTs and a voltage V will be applied on the outer pair of electrodes in order to force the electrodes to contact. This particular design has been devised in order to apply a lower voltage between the contact MWCNTs. For this particular design to work, the distance between the electrodes with CNTs, Int elec, has to be as close as possible and from a technological point of view, this will depend on the electron beam lithography step and in this case, has been assumed to be 100nm. Another parameter that needs to be considered is the length of contact between the CNTs, L cont. Upon actuation, the CNTs can be connected either by tip contact or by side contact. For this preliminary simulation studies, the length of the contact upon side-contact connection is assumed to be of the order of 116nm. This parameter has been obtained from Jang et al. [1,] and it has been included in a MATLAB simulation pack at TRT.. SIMULATION OF ELEMENTARY NEMS MECHANICAL SWITCH The key parameters identified for simulation purposes are : Young s modulus for the CNTs, E Diameter of the carbon nanotubes, ϕ cnt Length of the CNTs, L Interspacing between the electrodes with CNTs, Int elec Interspacing between the CNTs, Int cnts 10

11 11/17 For the sake of the simulation, F elastic, has been studied. Based on the literature, this parameter depends on the Young s modulus of elasticity, on the diameter and on the length of the CNTs as well as on the initial distance between the two contacted CNTs. The variation of each of these parameters and the impact on actuation voltage has been modelled and the results are described in the following sections...1 Variation of the Young s Modulus, E To assess the impact of E, the parameters below have been set : Length of contact, L cont : 116nm Diameter of the CNTs, ϕ cnt : 65nm Interspacing between electrodes, Int elec : 100nm Distance between CNTs, D cnts : 85nm Figure 5 shows the variation of the F elas /F vdw ratio with E at 0.8TPa, 1TPa and 1.TPa for various lengths of the CNTs. The point is to identify the impact of E on the length of the CNTs for a ratio F elas /F vdw > 1, which is the condition required for the CNTs to switch and release. A safe assumption would be to take this ratio to be superior or equal to 1.1. F elas / F vdw Length of CNTs (µm) Figure 5 : influence of the Young modulus on CNT length vs reversibility criteria From Figure 5, the variation of E in the range 0.8TPa to 1. TPa has a direct impact on the ratio F elas /F vdw and hence impacts the minimum length required for the CNTs. Based on the length obtained from this graph, the impact of E was then assessed on the actuation voltage, V. On Figure 6, the variation of V against L is illustrated. Note that the actuation voltage, V, is defined as the voltage to be applied to the outer pair of electrodes to force contact between the inner pair. 11

12 1/17 Actuation Voltage, V act (V) Length of CNTs (µm) Figure 6 : influence of the Young modulus on CNT length vs actuation voltage From Figure 5 and Figure 6, it can be deduced that the lower the length of the CNTs, the higher the spring force and the higher the Young s modulus the higher the spring force. Consequently, for higher Young s modulus we can have longer CNTs and maintain the reversibility and a close actuation voltage range. E 0.8 TPa L µm 1 TPa.15 µm 1. TPa.3 µm V actuation 40 V 39 V 37 V Table 1 : (E,L) couples for similar actuation voltages These results show that the 4electrodes configuration is a viable technological solution, with respect to the fact that it is hence possible to apply a minimum voltage between the commuting CNTs to avoid any breakdown upon switching... Variation of the contact length between CNTs, L cont To evaluate the impact of the length of contact, these parameters have been set for simulation purposes : Young s modulus for the CNTs, E : 1TPa Diameter of the CNTs, ϕ cnt : 65nm Interspacing between electrodes, Int elec : 100nm Distance between CNTs, D cnts : 85nm Figure 7 shows the variation of the ratio F elas /F vdw with respect to L, the length of the CNTs for different lengths of contact, L cont. This result is fundamental to the understanding of the geometry to be defined for the NEMS devices. 1

13 13/17 F elas / F vdw Length of CNTs (µm) Figure 7 : influence of L cont on CNT length vs reversibility criteria The length of contact has a direct incidence on the Van der Walls forces, the higher the contact length, the higher the VdW forces. With regards to the direct impact on the reversibility criteria, and as shown on Figure 7, it is necessary to increase the spring force as the contact length increases...3 Variation of diameter of CNTs and distance between CNTs To evaluate the impact of the diameter of the CNTs, these parameters have been set for simulation purposes: Young s modulus for the CNTs, E : 1TPa Length of contact, L cont : 116nm Interspacing between electrodes, Int elec : 100nm Figure 8 and Figure 9 show the variation of F elas /F vdw with respect to the length of the CNTs and the variation of the actuation voltage with respect to the length of the CNTs for F elas /F vdw = 1.1 respectively. The results of these variations are then summarized in Table below. 13

14 14/17 F elas / F vdw Length of CNTs (µm) Figure 8 : influence of external diameter on CNT length vs reversibility criteria Actuation Voltage, V act (V) Length of CNTs (µm) Figure 9 : influence of external diameter on CNT length vs actuation voltage MWCNTs external diameter 65 nm 80 nm 95 nm MWCNTs initial distance 85 nm 30 nm 355 nm MWCNTs length.15 µm.8 µm 3.4 µm MWCNTs actuation voltage 39 V 37 V 36 V Table : variation of CNT length, external diameter and CNT initial distance for similar actuation voltages As a conclusion of this work at TRT we have determined variation range for critical physical and design variables of the CNT actuator as well as proposed a new design of CNT actuator that enhances reliability of actuation and life time. 14

15 D. : Report on design of the 3 15/17 RF NEMS DESIGN AND SIMULATION 3.1 RF NEMS DESIGN Based on the previous study about the NEMS switches and depending on the results obtained, we could determine the RF NEMS design. Figure 10 : Design for the RF NEMS switch 15

16 16/17 3. RF NEMS SIMULATION Based on this design, we simulated the devices behavior. The first step is to simulate the behavior of the coplanar line. The results are shown in Figure 11 (a) (b) Adaptation (db) Fréquence (GHz) W=3µm et cpw=4µm W=36µm et cpw=7µm W=39µm et cpw=30,5µm Pertes d'insertion (db) -0,6-0,7-0,8-0,9-1 -1,1-1, Fréquence (GHz) W=3µm et cpw=4µm W=36µm et cpw=7µm W=39µm et cpw=30,5µm (c) Figure 11 : (a & b) molydenium coplanar line design, (c) return loss, (d) insertion loss We have also measured the S11 and S1 parameters in the off/on state of these RF NEMS ohmic swithes. The results are shown in Figure 1. (d) 16

17 17/17 Figure 1 : S11 and S1 parameter in the OFF/ON state of RF NEMS 4 CONCLUSION We have presented the design of a NEMS switch based on vertical CNT array using realistic values of nanomechanical parameters. The complexity of this device is beyond SoA. The actuation time is estimated to be about 1 ns and this is well below MEMS with 1- orders of magnitudes. An innovative bias and polarization techniques were also presented to avoid the CNT shortcuts during actuation. 17