Graphene/MoS2 based RF-NEMS Switches for Low Actuation Voltage and Enhanced RF-Performance Aakif Anjum, Mukesh Madhewar, Suhas S. Mohite Vishram B. Sawant Mechanical Engineering Department, Government College of Engineering, Vidyanagar, Karad, Maharashtra, India, 415124 [email protected] [email protected] Mechanical Engineering Department, Rajiv Gandhi Institute of Technology, Versova, Andheri (W), Mumbai, Maharashtra, India, 400097 [email protected] Abstract—In this work, the modeling, simulation and analysis of a contact type RF nano-electromechanical switches (RF-NEMS) with very low actuation voltage and enhanced RFPerformance is presented. The switches are modeled using previously known theory from literature and then optimized geometrical dimensions for low actuation voltage, low insertion loss and high isolation as objectives. The effects of the length, width and thickness of Graphene/MoS2 beam on various performance parameters are studied in details. Further, modal analysis, force, capacitance, release time, actuation voltage and S-parameters are computed using ANSYS and HFSS software’s. The switch exhibits low actuation voltage <1V for different thickness of Graphene/MoS2 as a beam material. The mechanical resonant frequency and quality factor are 72.5 kHz, 28 kHz and around 2, respectively, with a simulated switching time of 19 µs to 71 µs for optimum length of 10 µm are obtained for all three layers with comparison of results. It is concluded that low actuation voltage NEMS switches can be realised using single/bilayer layer 2D material with enhanced RF performance. Keywords—Graphene, MoS2, RF-NEMS, Actuation voltage, Insertion loss, Isolation. I. INTRODUCTION A Nano-electromechanical system (NEMS) is an emerging field for future technology development. Radio frequency based NEMS switches in future will be widely used over MEMS switches to further reduce voltage, low power consumption and to enhance RF performance [1]. The main reason is that RF-NEMS switches have low resistive loss, high isolation, low noise, low actuation voltage. RF-NEMS switches work on a wide range of frequencies for various applications such as wireless communications, satellite system, cell phones, highly sensitive Sensors, military applications, nano tweezers etc. [1-3]. There are various 2D materials which are used in nanofabrication of devices to name a few are Graphene, MoS2, borophene, germanene, silicene, stanene, phosphorene[1-3].These materials can be used as a bridge material in RF-NEMS switches by [3-5]. RF-NEMS switch XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE with two layer of graphene as a bridge material is presented by Milaninia et al. [3]. The dimensions used for beam is 20 × 3 µm (L × w) and g = 500 nm. The pull in voltage is 4.5 V is obtained. The drawback of this switch is the limitation of contact resistance (200 kΩ). This is because of non-uniform surface of graphene due to chemical vapour deposition process. Fixed-fixed type RF-NEMS switch consisting of graphene beam is modeled and simulated by Dragoman et al. [4]. CPW transmission line is formed using 20 nm gold which is patterned on silicon. The voltage obtained by this switch is 2 V.Graphene based RF-NEMS switches are modelled and analysed for fixed-fixed beam by Pankaj et. al. in 2014 [5]. The main disadvantage of all above switches is the actuation voltage’s are > 1V.Proper selection of 2D beam material so as to give minimum voltage is still challenging task and switch design has to be optimised at initial stage of design before fabrication. Reliability issues and its effects are not considered in this work. This motivates us to design RFNEMS Switch in order to get the minimum actuation voltage with low insertion loss and high isolation at various operating frequencies. II. SCHEMATIC Figs. 1(a) and 1(b) represent the up and down state position of graphene/MoS2 based cantilever type RF-NEMS switch. (a) (b) Si/SiO2 Gold Si3N4 Graphene/MoS2 Layer Fig. 1. Graphene based RF-NEMS switch (a) Up state (b) Down state In schematic representation above three elements are common in all radio frequency switches. One is transmission line which transmits the radio frequency signals from one port to another port that is from input to output port. The second is substrate, beam which is moving element generally cantilever or fixed-fixed beams are used for the actuation and un-actuation of the switch. Due to supplied voltage the electrostatic charges are developed which generates various forces at nanoscale which causes the movement of beam. In this study the thickness of beam is varied from 0.34 nm to 2nm while length and width is varied from 5 to 30 um to achieve set objectives of design. III. (1) Here EI represents effective bonding rigidity of the beam. Deflection of beam is denoted by ω, x is the distance measured from fixed end. Felec, Fc represents the electrostatic and casimir force per unit length. While Fs represent elastic force. According to fringe field of first order, F elec per unit length of cantilever beam is [6] ( )] [ ( ) ] (2) Where b represents width of beam, g0 is the distance between cantilever beam and central conductor ϵ0 is the permittivity of vacuum and V is the applied voltage. The force which is due to attraction between atoms is known as casimir force. The casimir force per unit length of the cantilever beam is [6] (3) * (4) This force may soften the cantilever type of switch. By combining Equation 2,3,4 The governing equation will be [6] ( ) ( ) [ ( )] [ ( ) ( )] ( )+ Here c is the speed of light that is 2.998 × 108 m s−1, h is planks constant/2 π. Material property does not affect the casimir force. XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE (5) ( ) [ ] (6) , ( ) [ ( ) MODELING A. Mechanical Modeling The mechanical modelling of RF-NEMS switch is involved by the three types of forces. The electrostatic force which acts due to the electrostatic charges induced between ground conductor and graphene beam. The second is elastic force, spring is modeled with the help of elastic force. This electrostatic force depends on size, shape and material of beam. The third is intermolecular forces. These forces include van der waals and casimir forces. The consideration of van der waals force is only when gap height g is less than 20nm. Practically fabrication of switch with gap height less than 20nm faces difficulties. Packaging and population becomes difficult to control. Due to this reason we have considered the casimir force and designed the model with gap height greater than 20nm. The consideration of casimir force is only when gap height g is greater than 20nm [6]. The governing equation of cantilever type switch is given as [6] ( ) Perpendicular load which acts due to small amount of deformation in a beam is elastic or transverse force. This Force is as [6] Modeling of series type RF-NEMS switch can be done by considering the capacitor which are parallel to each other. The actuation voltage for cantilever type RF-NEMS switch is as [7] √ ( ) (7) Where, gair=Air gap, εo=Permittivity of free space, w= Cantilever width, W= Length of CPW, K= Stiffness of spring The actuation voltage is dependent on stiffness and is given ( ) as [7] (8) Where W, t, L is width, thickness and length of beam, E is Young’s modulus of the beam. Mechanical quality factor is amount of energy stored divided by amount of energy released per cycle at the resonant frequency. This mechanical quality factor is important because higher the value of quality factor lower be amount of energy loss. The frequency (f), quality factor (Q) and dynamic viscosity (µ) are given as [8] √ √ ( (9) 10) ) √ ( ) (11) The switching and release time also important parameters of the RF-NEMS switch. The time taken by beam to come from up state to down state position is known as switching time. Switching time is as [5] (12) Vs=1.3* Vpull-in (13) Where, is the angular frequency. Release time of switch is given as [7] (14) Thus from above equation it is clear that release time is inversely proportional to frequency. Zs=Rs+jωL (22) ( (23) and for Zs=Rs, (ωL<<Rs) ) B. Electrical Modeling The series type RF-NEMS switch is modeled electrically. This model depends on C, R and L. where C represents capacitance. As there are two states up and down so there are two values of capacitance. Capacitance in up state denoted by Cu and the capacitance in down state is denoted by Cd. R and L is the resistance and inductance respectively. 1) capacitance: The up state capacitance is combination of parasitic capacitance denoted by Cp and series capacitance denoted by Cs. Thus up state capacitance is given by [7] Hence it is very simple to determine the series resistance with the help of reflection coefficient. The down state inductance can also be calculated. In this case reflection coefficient is fixed with RL model impedance is given as [7] And for Zs=jωL, (ωL>>Rs) (15) The series capacitance is the sum of parallel plate capacitance (Cpp) and fringing component Cf which is 0.3-0.6 of Cpp. Cp can be calculated using simulation software such as Sonnet, IE3D and HFSS. The parallel plate capacitance is given by [7] C. Material Properties of Graphene and MoS2 Graphene, which is a typical flat monolayer of carbon atoms arranged in a honeycomb lattice and has great interest in electronic devices [1]. It was first mechanically exfoliated from graphite in 2004 [1]. Molybdenum disulphide is silvery black two dimensional material can also be used for switching of RFNEMS switches. MoS2 is an inorganic and comes in a class of transition metal. It looks like graphite but same plane is not shared by MoS2 as graphene shares the same plane. This is because the graphene’s orbital are planer. Monolayer MoS2 is a semiconductor with a direct bandgap of 1.8 eV. This property of MoS2 will largely compensate the weakness of gapless graphene, thus next generation switching devices can be fabricated from them. (16) In above equation td/εr is because of dielectric thickness. For Si3N4 the relative dielectric constant is 7.6. The down state capacitance is given as [7] (17) The capacitance ratio is nothing but ratio of down state capacitance to the up state capacitance. [7] Cr=Cd/Cu (18) In order to get lower insertion loss and higher isolation capacitance ratio should be high. Capacitance ratio is high only when capacitance in down state is high and capacitance in up state is low. 2) Scattering parameters:The scattering parameters can be calculated with the help of up state and down state capacitance in off and on states. These scattering parameters will help in extracting the CLR values. In up state position, insertion loss and isolation can be calculate as [7] (19) (20) For extracting CLR value the effect of inductance and capacitance is not considered, as in case of up state effect of L and R is negligible. Loss of cantilever NEMS switch can be calculated with the help of extracted values of S11 and S12 [7]. Thus Loss= 1-|S11|2-|S12|2 (21) In case of down state resistance and inductance can be calculated with the help of reflection coefficient. The equivalent series impedance for down state position is as [7] XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE ( ) (24) Thus from above we can measure inductance in down state. TABLE I Material properties of Graphene and MoS2 Reference Graphene MoS2 p-doped p-doped [5] Doping Single layer thickness (SL) Bilayer thickness (BL) Multilayer thickness (ML) Young’s modulus Density Poisson’s ratio IV. 0.34 nm 0.3 nm [5], [11],[13] 0.64 nm 0.6 nm [5], [11],[13] 2 nm 1.7 to 1.8 nm [5] 0.3 Tpa [5],[10] 0.8 to 1 Tpa 3 2200 k.g/m 0.4 3 5060 k.g/m 0.125 [5],[10] [5],[10] RESULTS AND DISCUSSIONS A. Modal Analysis In an attempt to study the effect of the cantilever beam length, width, and thickness variations, the simulations of resonant frequency and deflection of cantilever beam were carried out. Each geometrical parameter of the micro structure is varied one at a time in order to interpret its effect on the frequency and deflection of cantilever beam. The Fig. 2 shows the cantilever type of graphene/MoS2 based RF-NEMS switch. Resonant frequency of the switch is obtained with the help of modal analysis. Resonant frequency of Graphene based beam for first, second and third mode are 72.5 KHz, 299 KHz and 582 KHz respectively while for MoS2 based beam 28, 131 and 227 KHz respectively. switches is studied. Both casimir and electrostatic force increases nonlinear with respect to deflection. Fig. 3 and 4 shows the nonlinear effect of casimir as well as electrostatic force. The length at which cantilever beam will not adhere to the surface due to casimir force is known as detachment length. Detachment length of 1.5 um is noted. All RF-NEMS switches require large down to up capacitance ratio of the State's ability to maintain high isolation and low insertion loss. Capacitance ratio of 1.59e-9 F is noted. C. Pull in voltage, quality factor, switching time: Analytical and simulation results are performed to study the mechanical and electrical parameters. B. Force and capacitance 13 40 tr for 0.34nm 35 tr for 0.64nm 30 tr for 2nm 7 20 5 15 3 0 -1 6 8 10 12 Length (um) Fig. 5. Release time and stiffness as a function of length for different values of beam thickness 0 2 4 From the graph it is clear that release time is inversely proportional to voltage. And it is observed that the release response time for 0.34 nm is 27% more than that of 0.64 nm. 20 100 200 300 tr for 0.34nm tr for 0.64nm tr for 2nm 18 Deflection (nm) Release time (us) 16 Fig. 3.Casimir force as a function of deflection 20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 100 90 80 14 70 12 60 10 50 8 40 6 30 4 20 2 10 Pull-in voltage(mv) Casimir force (µN) 1 5 0 Electrostatic force (µN) 9 25 10 50 45 40 35 30 25 20 15 10 5 0 11 Stiffness (uN/m) Fig.2. Modal patterns of the beam structure for Graphene/MoS2 Release time (us) 45 0 0 5 7 9 11 Width (um) Fig. 6. Release time and voltage as a function of width for different values of beam thickness 1 0 100 200 300 Deflection (nm) Fig. 4.Electrostatic force as a function of deflection Casimir effect as well as electrostatic effect on the critical pull-in gap and actuation voltage of nanoelectromechanical XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE 3 Fig.6 shows the variation of release time and voltage with width variation the optimum width (3um) and the thickness t=0.34 nm, 0.64 nm and 2nm respectively. The dynamic response shows that the fast switching that is less than 10 us for NEMS Switch with width 3 um shows actuation voltage of (<2mv). 35 10 11 9 30 25 7 20 5 15 3 10 6 4 2 1 5 0 -1 5 7 9 11 Length (um) Fig. 7. Release time and voltage as a function of length for different values of beam thickness 0 Fig.7 shows the variation of release time and voltage with length variation the optimum length (10um) and the thickness t=0.34 nm, 0.64 nm and 2 nm respectively. The dynamic response shows that the fast switching that is less than 20 us for NEMS Switch with length 10 um shows actuation voltage of (<2mv). 160 3 Switching time (us) 13 ts for 0.34nm 70 ts for 0.64nm 60 ts for 2nm 11 9 50 7 40 5 30 3 20 1 10 0 -1 1 3 5 7 9 11 Length (um) Fig. 8. Switching time and voltage as a function of length for different values of beam thickness From the graph it is clear that switching time is inversely proportional to voltage. And it is observed that the switching response time for 0.34 nm is 28% more than that of 0.64 nm. As the quality factor of the NEMS switch depends upon the cantilever length, width, in Fig. 9, the effect of geometric dimensions on quality factor of the RF-NEMS switch is plotted with the cantilever length, width as the variables for thickness of 0.34, 0.64 and 2 nm. The results XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE 0 10 20 30 40 Length (um) (a) 140 Quality factor 80 Pull-in voltage(mv) 1 Q for 0.34nm Q for 0.64nm Q for 2nm 8 Quality factor 40 Release time (us) 13 tr for 0.34nm tr for 0.64nm tr for 2nm Pull-in voltage(mv) 45 show that the quality factor decreases for a given cantilever length and width. Q for 0.34nm 120 Q for 0.64nm 100 Q for 2nm 80 60 40 20 0 0 10 20 30 40 Length (um) (b) Fig. 9. Quality factor of beam as a function of (a) length (b) width Fig. 9(a) shows the decline of quality factor with increase in length. For thickness of 0.34 nm at optimum length of 10 um the quality factor is 0.4. While for 0.64 nm thickness it is 1.6. The quality factor for 2 nm thickness is 15 for 10 um length and 1.7 for 30 um length. In Fig. 9(b) at optimum length, quality factor of 1 and 3 for thickness of 0.34 nm and 0.64 nm respectively noted. The results show that the variation in the micro beam width has comparatively greater effect on the quality factor of the switch than the beam length. For better performance of cantilever switch, actuation voltage should be low and quality factor should be high. By comparing calculated performance parameter values of actuation voltage of graphene is low and quality factor is greater than others. If actuation voltage is high, then a chance of switch failure increases. D. Insertion loss, Isolation of RF-NEMS Switches: The S-parameters in the down state position is shown in Figs.10 and 11. The isolation and insertion loss values are obtained by simulations using ANSYS HFSS (High Frequency Structural Simulator) for cantilever type RFNEMS switches. The isolation and insertion loss values obtained are in the range of 62-69 dB and 0.0342-0.0379 for graphene and MoS2 switches. The isolation 69 dB for bilayer graphene and 63 dB for bilayer MoS2 are obtained. The bilayer of graphene and MoS2 switch offers a superior isolation as compared to the monolayer and multilayer because of the lower surface resistance. TABLE II Comparisons of Graphene and MoS2 based RF-NEMS Switch Ref. Milaninia et al. 2009 [3] Dragoman et al. 2009[4] Material Pull-in voltage, Vpi(V) Graphene 4.5 V Graphene 2V SLG 0.3 V MLG 1.4 V SLG 1.38 mV BLG 3.56 mV MLG 19.71 mV SLG 0.543 mV BLG 1.4 mV MLG 7.75 mV Isolation -30dB at 60GHz Pankaj et al in 2014 [5] Fig. 10. S11 and S12vs frequency in down state position of the NEMS switches for graphene Proposed MoS2 VI. Fig. 11. S11 and S12vs frequency in down state position of the NEMS switches for MoS2 V. COMPARISION We have further studied the MoS2 based RF-NEMS switch and compared with the graphene based RF-NEMS switch. These Graphene/MoS2 based switches are compared with the work done by various researchers previously. From TABLE II, it is noted that Graphene/MoS2 based switches show low actuation voltage, low insertion loss and high isolation than that of work done by other researchers. The cantilever type RF-NEMS switches show good performance in terms of electrical and RF characteristics. XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE ---------- -40dB at 60GHz >10db @ Proposed Graphene Insertion Loss ---------- 1-60 GHz 0.01–0.3 dB >20db @ 0.01–0.2 1-60 GHz dB -65.3441 -0.0352 at at 9GHz 10GHz -69.2384 -0.0344 at at 7GHz 9GHz -62.674at 7GHz -62.38 at 11 GHz -63.43 at 17 GHz -62.9968 at 7GHz -0.0379 at 14 GHz -0.0342 at 9 GHz -0.0374 at GHz -0.0379 at 14 GHz CONCLUSIONS The performance of cantilever type RF-NEMS switch based on 2D material is calculated and by comparing the performance parameters viz., actuation voltage and quality factor, insertion loss and isolation are computed using simulation. The switch exhibits low actuation voltage <1V for different thickness of Graphene/ MoS2 as a beam material. The mechanical resonant frequency and quality factor are 72.5 kHz, 28 kHz and around 2, respectively, with a simulated switching time of 19 µs to 71 µs for optimum length of 10 µm are obtained for all three layers with comparison of results. It is concluded that low actuation voltage NEMS switches can be realised using single layer 2D material with low insertion loss. ACKNOWLEDGMENTS The support and financial assistance received through TEQIP program at CIM Laboratory of Mechanical Engineering Department, Government College of Engineering Karad. Thanks to Senior Facility Technologist at CeNSE, Indian Institute of Science, Bangalore, for interaction related to design and fabrication issues with 2D materials. REFERENCES [1] [2] [3] [4] [5] [6] [7] Yasser Mafinejad, Abbas Kouzani, Khalil Mafinezhad, “Review of low actuation voltage RF MEMS electrostatic switches based on metallic and carbon alloys” Journal of MicroelectronicsElectronic Components and Materials Vol. 43, No. 2,2013. 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