Design optimization of RF MEMS meander based ohmic contact switch in CPW and microstrip line implementation

Transcription

1 Proceedings of ISSS 28 International Conference on Smart Materials Structures and Systems July 24-26, 28, Bangalore, India ISSS-28/SX-XX Design optimization of RF MEMS meander based ohmic contact switch in CPW and microstrip line implementation P. Boora a, Savita Maurya a, N Girdhar a, K. Maninder b, K. J. Rangra b a Dept. of AIM & ACT, Banasthali Vidyapith. b Sensors and Nanotechnology Group, CEERI Pilani , Rajasthan, kjrangra@gmail.com, erpoonamboora@gmail.com, mauryasavita5@gmail.com ABSTRACT In this paper, the design optimization of RF MEMS meander based ohmic contact switches for S and C band applications using CPW and micro strip line is presented. The switches designed for 2-8 GHz frequency range have analytically calculated actuation voltages in the range of 1 to 2 volts. In CPW configuration, the simulated insertion loss and isolation are.2 to -.25 db and -51 to -3dB respectively, for frequency range 2 to 8 GHz, while in microstrip media, the corresponding s-parameters are -.1 to -.4dB and -59 to -38dB, for 2-8 GHz. In the present work, both the approaches i.e. CPW and microstrip have been considered to optimize the device topology and overall size. Keywords: RF MEMS, CPW, microstrip, ohmic contact switch, insertion loss, isolation I. INTRODUCTION MICROELECTROMECHANICAL switches present a promising technology for high-performance reconfigurable microwave and millimeter-wave circuits. The low insertion loss, high isolation and excellent linearity provided by MEMS switches offer significant improvements over the electrical performance provided by conventional PIN diode and MESFET switching technologies[1]. In terms of performance, the RF switches have demonstrated both low on-state insertion loss and high off-state isolation, using coplanar waveguide (CPW) and microstrip connector configurations, as compared to state-of-the art solid state devices. Therefore, RF MEMS switches are an attractive solution to switch antenna bands and transmit/receive switching for future multi-band, high bandwidth cell phones and satellites etc. This paper focuses on the design and simulations of meander based ohmic contact switches in CPW and microstrip line implementation. CPW transmission lines have been widely used in recent years as feeding networks with slot antennas [2]. In comparison to microstrip line, CPW has advantages like low radiation loss, less dispersion, and uniplanar configuration. It also facilitates easy mounting and integration with other microwave circuit components. In general CPW lines are designed with characteristic impedance of 5Ω. It allows both series and shunt circuit elements to be easily incorporated into the circuit without via holes or wrap around wires. Therefore, coupling between CPW and other circuit elements is reduced which permits closer line spacing and smaller sizes. The propagation constant and characteristic impedance of CPW are not strongly dependent on the substrate thickness [3]. The CPW topology under consideration and its RF response are shown in figure 1. Microstrip transmission lines are also used for microwave circuit design, though sufficient references are not available for direct implementation in MEMS switching circuits. In comparison to CPW, the microstrip transmission lines are simpler to fabricate and allow compact sizes. The radiation loss for same area are relatively higher but get compensated by low insertion loss as microstrip lines utilize less area co- Further author information: (Send correspondence to K.J.Rangra) P. Boora: erpoonamboora@gmail.com, Telephone: K.J.Rangra: kjrangra@gmail.com, Telephone: , SNG, CEERI, Pilani, Rajasthan, India 1

2 Figure 1. Side view of CPW RF response of CPW. Figure 2. Side view of microstrip RF response of microstrip. -mpared to the CPW. The microstrip designs not only offer easier biasing of the individual switches especially if switches are used in large numbers, but also provide area compactness. However, it requires via-hole technology for obtaining a wideband short circuit to ground which creates fabrication complexity [4]. The microstrip line topology considered for the present work is shown in figure 2 along with its RF response. II. SWITCH DESIGN CPW Implementation As mentioned earlier, the proposed SPST ohmic switches are built in CPW transmission lines (S/W/S = μm) with characteristic impedance of 5 Ω. As shown in figure 3, the switch is anchored to the substrate by four serpentine springs, used to substantially lower the switch stiffness. Each meander of the spring is defined as a set of four beams, two primary beams and two secondary beams. The change in spring constant of the structure using suspension springs offers more flexibility since it does not considerably impact the size, weight and RF performance of a switch [5]. The working principle of a RF switch resembles the electro-mechanical relay"[6]. In series ohmic configuration instead of the `capacitive overlap' of the bridge and transmission line, an ohmic contact flexure as shown in figure 3 is used to transmit the signal over a gap of 15 μm in the transmission line. The contact flexure is suspended above the interrupted transmission line by a vertical gap height of 3μm. This combination offers an excellent isolation in the switch off-state. The performance of such a configuration is mainly characterized by the transmission loss; high isolation is comparative- 2

3 Figure 3. 3 D model of ohmic switch in CPW configuration (CoventorWare ) and 3 D view of ohmic switch in microstrip configuration using 3D EM Solver. -ly easy to obtain because of the larger air gap in the transmission line and the vertical air gap between the contact flexure and the contact points on the transmission line [5]. This off-state position of the switch provides high isolation (better than -28dB) for dc to 1 GHz. Under the actuation bias, the contact flexure falls onto the transmission line and creates a short circuit by joining the open ends of the t-line. This corresponds to the switch on-state. As shown in figure 4, low loss electrical contact is provided by contact bumps placed at the end of the interrupted portion of t-lines. Bumps are provided in case of series switch to make an effective contact during the down position of the beam. The most critical aspect of the switch is the optimization of the transmission characteristics, which are affected by the loss either due to the leakage towards ground or series resistance and the contact resistance (bump - t-line contact). Both aspects have been accurately modeled and minimized. The contact resistance is a function of the contacting metal properties, contact force and contact area. A detailed contact resistance model is given in [7]. Following the approach, the contact flexure design as shown in figure 4 consists of a row of bumps on the t-line and a compliant contact flexure fixed to the rigid bridge plate. The mechanism ensures a point-like elastic contact between the bumps and the bridge with a controlled amount of force. The bumps (5μm 5μm) are defined in polysilicon, under the underpass area of the t-line. The metallic (Au) switch membrane has length, width and thickness of 15, 9, and 1.5 μm, respectively. Figure 4. Schematic cross-sectional view of the flexure equivalent circuit of a series switch in CPW. 3

4 The pull - down electrodes are connected using polysilicon bias lines up to the edge of the ground plane of the CPW for easy access.the electrical equivalent circuit of a series switch is shown in figure 4. The RF response of the device is mainly characterized by the switch capacitance in up-state and the inline segment resistance which also includes the contact resistance, in the down-state. As shown by the equivalent circuit the up-state capacitance is composed of series capacitance C s, the capacitance between the flexure and transmission line overlap area, parasitic capacitance between the open ends of the t- line. So that the total capacitance is: Cs C = + C (1) 2 up The series capacitance, C s arises because of the overlap of metal portions of the t-line and the flexures, separated by an air gap (3 μm) and it can be estimated as a parallel plate capacitance given by C pp = εa/g. The parasitic capacitance C p depends on the CPW width and the separation between the two ends of the t-line. In the on-state, the switch is simply a continuation of the CPW t-line and the insertion or transmission loss is mainly due to the contact resistance (R c ) and the resistance of the inline segment (R l ). The total resistance of the switch thus, can be written as, [5] R s = 2 Rc + R (2) l Microstrip line Implementation The absence of adjacent ground plane in microstrip line results in higher radiation losses compared to CPW with similar dimensions. As described in [4] extra radiation loss can be obviated with careful design considerations. Figure 3 shows 3D view of microstrip series switch. Figure 5 shows equivalent circuit model of microstrip series switch. The t- line portion of switch is modeled by π-equivalent circuit which is further represented by elements L a, L b, R a, R b C a, and C b. The bridge segment which makes contact with the open ends of t-line in the switch on-state is modeled separately in on and off states. R C1 and R C2 are contact resistances of bridge and depend on the size of the contact area, the mechanical force applied, and the quality of the metal-to-metal contact. The contact resistance has finite value when switch is on- state and zero in the off-state. In the switch off-state, bridge is suspended above the t-line gap, and is modeled by capacitances which also include the fringing field component of capacitance C off1 and C off2. Capacitance C up shown in the equivalent circuit is due to the gap between open ends of t-line and contact portion of bridge. In the onstate it simply is a continuation of the t-line and is modeled by π-equivalent circuit which includes down state capacitances C on1 & C on2, series resistance R on and bridge inductance L on. R via and L via represent resistance and inductance contribution due to via holes respectively which provide ground to the switch. Current in microstrip switch is concentrated on the edge of the bridge over the t-line gap and takes the shortest path along the edges of the line to the anchor portion. The insertion loss of switch in on-state is dominated by the resistive loss of the t-line and the coupling p Figure 5. Equivalent circuit of microstrip switch. 4

5 between the t-line and the bridge.to minimize the resistive loss, t-line is build using a thick metal (Au) layer. The problem of skin effect needs heedful consideration when thickness of metal layers are considered; as the frequency increases the signal tends to reside on the outer periphery of the conductor. For the bridge thickness smaller than two skin depths, the switch resistance is constant with frequency. For Au-bridge thickness >1.5µm, the switch resistance changes with under-root of frequency, above 1GHz due to skin depth effect, where as for thin Au bridge (.5-1 µm), the bridge resistance is constant up to 3 GHz [4]. A gap is created in the microstrip t-line when the switch is in offstate resulting in high isolation. In the up-state position the switch isolation is limited by the up state capacitance [4]. III. EM Simulations RF Response of switch in CPW Line Electromagnetic characterization of the switch for lever lengths of 4 μm, 45 μm, and 5 μm has been carried out using commercial 3D-electro-magentic solver, with two-port s-parameters measured from 1 to 1GHz. Figure 6 and 7 show the RF response of switch in off and on states for different lever lengths. In switch off state isolation and in onstate insertion loss are considered whereas the return loss is important in both the cases. As observed from figure 6 and 7 the switch with lever length 45 μm shows better isolation (-31 db up to 1 GHz) as compared to the other two switches utilizing lever lengths 4 μm and 5 μm. The overall simulated RF response of series switch in on-state (insertion loss) is less than -.25 db and in off-state (isolation) is more than -3 db. Isolation(dB) Off State db(12)4) db(s(12)45) db(s(12)5) 4um 5um Return Loss(dB) Off State db(s(11)4) db(s(11)45) db(s(11)5) 4um 5um Figure 6. RF Response of switch in off-state for different lever lengths (4 μm, 45 μm and 5 μm) Isolation Return loss. Insertion Loss(dB) On State db(s(12)4) db(s(12)45) db(s(12)5) 4um 5um Return Loss(dB) On State db(s(11)4) db(s(11)45) db(s(11)5) 5um 4um Figure 7. RF Response of switch in on-state for different lever lengths (4 μm, 45 μm and 5 μm) Insertion loss Return loss

6 RF Response of switch in Microstrip Line Figure 8 and 9 show the variation of s-parameters as a function of frequency for various lever lengths (L) of the switch (4 μm, 45 μm and 5 μm). The dependence of isolation on lever length is clearly shown by figure 8; better isolation characteristics are obtained at L = 45 μm. With L = 4 μm, the isolation characteristics are rather degraded as compared to L = 45 μm, further increase in L doesn t significantly improve the switch isolation. Isolation is more than -39dB for all the switches below 1GHz. Figure 9 shows response of the switch in on-state ; insertion loss also follows the same trend as isolation. It can be easily seen from figures 1, 11, 12 and 13 that switch in microstrip implementation shows less insertion loss in the on-state and better isolation in the off-state as compared to CPW implementation for all three types of lever lengths, showing that the overall performance of the ohmic contact switch in microstrip implementation is much better then the CPW implementation. Insertion Loss(dB) On State db(s(12)4) db(s(12)45) db(s(12)5) 4um 5um Return Loss(dB) On State db(s(11)4) db(s(11)45) db(s(11)5) 4um 5um Figure 8. RF response of switch in off-state for different lever lengths Isolation Return loss. Isolation(dB) Off State db(12)4) db(s(12)45) db(s(12)5) 4um 5um Off State Figure 9. RF response comparisons in on-state for different lever lengths Insertion Loss Return Loss. Return Loss(dB) db(s(11)4) db(s(11)45) db(s(11)5) 4um 5um

7 Isolation (db) Off state CPW, L= 5 µm L Lever length CPW, L=45 µm CPW L=4 µm Microstrip, L= 4 µm Microstrip, L= 5 µm Microstrip, L= 45 µm Figure 1. Comparison of switch in Microstrip and CPW implementation for different lever length in off-state (isolation). Return Loss (db) Off state CPW, L=4 µm Microstrip L= 4 µm Microstrip L= 45 µm L Lever length Microstrip, L= 5 µm CPW, L=45 µm CPW, L= 5 µm Figure 11. Comparison of switch in Microstrip and CPW implementation for different lever lengths in off-state (return loss)..5 On state Microstrip, L= 4 µm Microstrip, L= 5 µm Insertion Loss(dB) CPW, L=45 µm Microstrip, L= 45 µm CPW, L=4 µm -.2 L Lever length CPW, L= 5 µm Figure 12. Comparison of switch in Microstrip and CPW implementation for different lever lengths in on-state (insertion loss). 7

8 Return loss(db) On state L Lever length CPW, L= 5 µm CPW, L=45 µm CPW, L=4 µm Microstrip, L= 45 µm Microstrip, L= 5 µm Microstrip, L= 4 µm Figure 13. Comparison of switch in Microstrip and CPW implementation for different lever lengths in on-state (return loss). (d) Proposed Fabrication Process This section describes the proposed fabrication process in brief, details of the process are given in [8]. In the seven mask fabrication process; the movable Au bridge structure is micromachined in two electroplating steps by using commercial electrolyte solutions. High resistivity silicon wafers (p-type, 5kΩ) are preferred in view of the process compatibility with existing in-house facilities and low insertion loss requirements to fabricate the switches; optionally resistors and capacitors can also be fabricated in the same process sequence. Thermal oxidation (1 nm) of the wafer is followed by patterning of the actuation electrodes and biasing resistors in LPCVD deposited polysilicon. After a boron implantation and activation cycle at 925 o C for 6 min, a layer of TEOS SiO2 is deposited and patterned to open contact holes. The polysilicon layer is also used to build the actuation pad areas for better isolation. In the next step the underpass area is defined, which connects the input and output ports of central conducting line of CPW. The multilayer metal structure has sputter deposited out-diffusion capping layer of Ti, and TiN, low resistivity Al:Si and final smooth, capping layer of sputter deposited TiN. Total thickness of the multilayer equals the thickness of polysilicon. In the next step via holes are patterned in it. To build the suspended movable bridge membrane, a 3µm thick photo resist layer, hard backed at 2 o C, is used as a sacrificial layer. Next, a seed layer for electrochemical deposition of Au, a Cr/Au layer is deposited by PVD on which the movable membrane is patterned. This is followed by electrochemical deposition of 1.5 µm thick Au layer. The next photolithography defines the CPW lines and anchor posts for the bridge and is followed by the selective electrodeposition of 3.5 µm thick gold layer. This, second electroplating step increases the thickness of selected portions of the bridge to 5. µm. After the removal of last masking layers, the membrane is finally released using a modified plasma ashing process, to prevent stiction problems [8]. The fabrication process discussed above is for switch fabricated in CPW implementation, steps of fabricating the switch in micro-strip implementation are similar except the additional mask level for defining the via holes for ground connections. IV. CONCLUSIONS MEMS series switches for RF applications have been designed and simulated. In terms of RF performance the switches show low insertion loss in the on-state and high isolation in the off-state over a wide frequency range. CPW and microstrip implementation of the switch are presented and compared. An iterative comparison showing the effect of different lever length on the performance of the switch has been carried out and the length is optimized for 45µm, for both the implementations. From the comparative analysis for the two well known connector media it can be concluded that microstrip implementation not only shows better performance compared to CPW but also results in more compact sizes which assumes higher significance while implementing the switches in SPnT form. V. ACKNOWLEDGEMENT The authors are thankful to Dr Chandra Shekhar, Director, CEERI, Pilani for the support and encouragement. We are also thankful to Dr. Rekha Govil, Dean, AIM & ACT, Banasthali Vidyapith, Banasthali for providing the opportunity to work at CEERI, Pilani. 8

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