Design and Fabrication of Low-loss RF MEMS switches for a broadband reflectarray

Transcription

1 Design and Fabrication of Low-loss RF MEMS switches for a broadband reflectarray Afshin Ziaei (1), Thierry Dean (1), Michel Charrier (1), Paolo Bondavalli (1), Jean-philippe Polizzi (1) Hervé Legay (2), Béatrice Pinte (2), Etienne Girard (3), Raphael Gillard (3) 1 Thales Research and Technology France, Domaine de Corbeville, Orsay Cedex, France 2 Alcatel Space, 26 avenue JF Champollion, BP 1187, Cedex, Toulouse, France. 3 IETR/INSA 20 avenue des Buttes de Cœsmes Rennes Cedex France. Abstract : This paper presents the design, fabrication and testing of capacitive RF MEMS switches for microwave/mmwave applications on high-resistivity silicon substrate or glass. The feasibility study and demonstrator fabrication of a new concept of reflector network using MEMS switch based phase-shifters concept for space antennas is presented. These switches can be accurately modeled using 3-D static solvers. The loss in the up-state position is equivalent to the CPW line loss and is db at GHz. It is seen that the capacitance, inductance and series resistance can be accurately extracted from DC-40 GHz S-parameter measurements. The reflector array antennas utilization for phase control avoids the use of very expensive directive antennas and covers a very large frequencies range. We will deal with the configuration, the composition and arrangement of MEMS switches, used to control the phase shift of the electromagnetic wave reflected by each elementary cell. INTRODUCTION The use of micromachined switches for RF switching applications was first demonstrated in 1979 as electrostatically actuated cantilever switches [1-3]. Since then, a large amount of research effort has focused on the fabrication and the implementation of micromachined switches for various applications and specifications such as reflect array antennas or phase shifters. Despite the differences in designs, these switches have demonstrated low insertion loss, high isolation, and low return loss (good impedance matching) at microwave frequencies. 1. Design and fabrication of Low-Loss RF-MEMS Switches 1.A. Design and fabrication Fig. 1 shows a CPW shunt capacitive MEMS switch. The technology relies on a metal planar membrane suspended over the central line of a coplanar waveguide, and resting on its ends on the ground lines. An electrode and a dielectric are integrated in the signal line. When a DC voltage is applied between this electrode and the metal membrane, this latter is deflected downwards due to electrostatic forces. In the un-actuated state, the membrane presents a high impedance with respect to the line, and the signal is unaffected. In the down position, the membrane is capacitively coupled to the bottom plane, and short-circuits the signal to the ground. TRT ( THALES Research & Technology) MEMS switching technology employs a thin dielectric coating over the center conductor, so that the device essentially switches between two capacitance states (up and down state) Typically a 2000 Å thick silicon nitride film is used with a relative permetivity of 7. The capacitance in the two states can be accurately computed using parallel plate formulas, requiring only knowledge of the electrode geometries and the dielectric material characteristics. The MEMS developed at TRT used membrane switch type i.e., the metal plate suspended over the signal line is attached at both ends. This choice was made to have a stiffer structure so as to lower the electrostatic sticking risks. Compared to a cantilever beam switch type, the membrane type is more rigid, which allows shorter commutation time but as a drawback requires higher activation tension. Glass was chosen for the substrate as it

2 Fig 1. Top view of TRT shunt MEMS capacitive switch is of a moderate price and has a high resistivity. MEMS switches are made in seven micro-lithography and deposit/etch steps. After realising the membrane on an expendable slab, the last step is to get rid off this slab. Between two operating modes, a wet one and a dry one, we chose to use O 2 plasma that doesn t require a supercritical CO 2 dryer: If wet etching is used for sacrificial layer removal, the rinsing liquid causes the suspended structure to collapse and stick due to capillarity forces. Another advantage of the RIE 0 2 dry etching is to have a cooling substrate device. As the RIE etching is anisotropic, the membrane must but be drilled regularly. The holes diameter is 4 m and the drilling grid is 8 m. 1.B. Measurements Figure 2 reports the measured S-parameters in the 0-40 GHz frequency range, for the UP state and DOWN state of the switch. The measurements were made on switches embedded within a 50 ohms waveguide test structure, using a Wiltron vector network analyzer and a Cascade Summit RF probing system. UP state Insertion loss ,05-0, UP state Return loss Pertes A(dB) -0,15-0,2-0,25-0, ,35 F (GHz) (a) (b)

3 Isolation (db) DOWN state Isolation DOWN state S11 Return loss (c) Figure 2: Measured S-parameter of Shunt-MEMS switch: In (top a-b) the UP state and, in (down c-d) the DOWN state. In this configuration, the S21 measurement in the UP state can be interpreted as the 'INSERTION LOSS' of the switch and the S21 measurement in the DOWN state can be interpreted as the 'ISOLATION' of the switch. For 40 GHz switching operation, when the switch is in the UP state, the insertion loss (a) of the switch is less than 0.3 db with the return loss (b)better than 11 db; when the switch is switched to the DOWN state, the isolation (c) is 32 db. The required voltage to pull the membrane in the down position is 30 to 40 volts. (see geometrical parameters and microwawe characteristics of TRT shunt switch in Table1). (d) Table 1. Physical and geometrical parameters and microwave characteristics of TRT shunt switch The technology developed here was first validated with shunt switches. Afterwards a more complex system that use several switches in series was built. Since then our research effort has focused on the integration of MEMS switches in a reflectarray used as a spatial antenna. Reflectarray antennas with scanning ability are low cost compared to mechanically steerable antennas and exhibit low losses compared to direct radiating arrays. 2. Description and fabrication of the reflectarray antenna with TRT RF MEMS switches The studied reflectarray has to re-radiate an incoming circularly polarized wave in a chosen direction without inverting its polarization. The unit cell of such a reflectarray must therefore re-radiate an identical circularly polarized wave as the incoming wave while adding a phase shift depending on the position of the radiator in the cell [4,5].

4 The influence of geometrical parameters will be studied. An active cell operating in Ku band will be shown and the phase shift will be achieved by using MEMS switches developed for this work. 2.A. Operating mode of the structure. The studied passive cell is displayed in figure 3. The radiating element is embedded in a metallic cavity in order to reduce mutual coupling between adjacent cells. In the final version, the metallic cavity will be covered with a dielectric sheet to minimize the importance of incidence angle [6]. For a circularly polarized incoming wave, two degenerated modes will be induced in the cavity. The radiating element has to re-radiate those two modes with a phase difference of 180. To do so dipoles above a ground plane are used. A dipole reflects the mode whose electric field is parallel to it (along Y in the following) while the ground plane reflects the orthogonal mode. The distance between dipole and ground (e+h) is used to control the phase shift between the two reflected modes. The ability to control the phase of the resultant re-radiated circularly polarized wave is due to the combination of several dipoles (6 in the present work) with different rotation angles [4,5]. For a given phase, only one of the six dipoles is active (i.e. its two arms are connected together through the central disc, while the others are not connected to this central disc). MEMS switches are used to select one dipole among six as explained in paragraph 1A. 2.B. Description of the reflectarray antenna Figure 3. Studied structure The antenna proposed is presented in Figure 4. It consists of a 330 elements reflectarray, with an elliptical shape, positioned on an azimutal actuator. The array lattice is 0.7, where is the wavelength. The azimutal pointing of the beam is performed mechanically. The elevation pointing ( 25 ) and the shaping of the beam are performed electronically through 2.5 bits phase shift elements. The number of phase states (6) was optimized to comply with performance for minimum complexity.

5 Figure 4: Reflector array design 2.C General considerations of phase shifting elements There are two possibilities for designing phase shifting elements: For the first class of element, the radiating part and the phase shifter part are fully dissociated. The radiating element is a patch element coupled and matched to a transmission line. Switches are arranged on this transmission line so that different path lengths, and therefore different time delays, may be achieved ; For the second class of element, the radiating part and the phase shifting part can not be separated. The MEMS device is inserted within the element, and modify the antenna pattern, so that it is fully involved in the element behavior. The devices modify the element characteristics, and affect the phase of the wave reflected by the element. The second family is preferred to the first one, since : Better bandwidth may be obtained. There is no matching conditions of the radiating part to the phase shifting part, which limits the bandwidth. The characteristics of the whole element is considered and optimised ; The MEMS switches are implemented in the heart of the element. This should lead to an easier accommodation of the devices within the array lattice ; 2.D principle of the selected phase shift element One promising solution [7], described by Phelan in 1976, and adapted to circular polarization has been adopted. It consists in working with an array of identical elements which have undergone different rotations. The property used is as follows: The response of an element to a circularly polarized field depends on the orientation of this element [8]. If it is oriented by angle, the phase of the field reflected by the antenna element varies by 2. In this way we achieve broader band operation as the phase shift is not obtained through variation of the dipole resonance. The phase-shifting cell developed makes the most of this property. It is illustrated in Figure 5. Upper etching is composed of dipole elements set out concentrically around a central pellet. This etching is located at a quarter

6 wavelength from a ground plane. Two opposed dipole elements are then connected using low-loss switches to the centre pellet to create a resonating dipole. BF commands /4 ground plane Multi layer Figure 5: Six phase state cell The active dipole reflects the component of the incident field parallel to it, while the orthogonal component is reflected by the ground plane. A distance of a quarter wavelength between the etching and the ground plane guarantees circular polarization. The phase shift obtained then depends on the orientation of the dipole built. To break the coupling between dipoles which disturbs cell operation, the cells are inserted in a metallic cavity. Consequently, the field reflected by a cell is independent of the orientation of neighboring cells. The cell operates in guided mode. It is therefore considered to be independent of the angle of incidence of the illuminating plane wave. 2.E implementation with MEMS switches The layout of the reflectarray antenna is presented in Figure 6. It consists of the assembly of : an hexagonal metallic grid, which converts the incident wave into a propagating wave within a hexagonal waveguide section; an hexagonal integrated macro component including the dipoles elements and the MEMS switches ; a PTFE substrate in which via-holes are drilled, and on the top of which the hexagonal grid and the MEMS macrocomponents are reported. The via holes ensures the continuity of the waveguide section, and permits the distribution of the DC control signals to the MEMS devices ;

7 Centrage / Support de test Alignement / cellules Grille Pyrex Metclad Support et commandes Report 3 Report 1 Figure 6 : Implementation of the reflectarray Resistive lines connect theses via holes to the MEMS switches. High resistivity is required, so that it does not affect the characteristics of the phase shift elements. A design procedure, guided with a FDTD software, conducted to the final design of the phase shift elements. The constraints were : to locate the switches not too far from the centre of the phase shift element; to keep the largest value as possible for the MEMS characteristics, expressed as a Con/Coff ratio, where Con refers to the capacity with the membrane down, and Coff to the capacity with the membrane up ; to avoid narrow sections for the dipoles, which makes difficult its optimization ; Figure 7 : Description of a phase shift element Series of phase shift elements were manufactured, ranging from passive phase shift elements to a fully equipped phase shift elements (with MEMS control, polariation resistive lines, and protection). The individual effect of the different processes were individually characterized : effect of the protection, of the polarization lines, of the viaholes,

8 The active cell is displayed in figure 8. Phase variation of this cell (figure 9a) is stronger than the passive case. The insertion of MEMS switches and polarization lines perturbs a little the function of the structure. Nevertheless, the axial ratio (figure 9b) is less than 2dB on the whole frequency range [ GHz]. A full demonstrator with 313 phase shift elements (only a fraction of them are fully active cells) has been fabricated (figure 10). Figure 8. MEMS embedded in the structure Figure 9. Optimal case for an active cell

9 Figure10. Full demonstrator with 313 phase shift element CONCLUSION Measurements demonstrate that the MEMS switches with metallic membrane possess low insertion loss and good isolation at frequencies up into the millimeter-wave bands. These devices offer the potential for building a new generation of low loss high-linearity microwave circuits for a variety of reflectarray antenna or phased antenna arrays for radar and communications applications. An innovative reflectarray antenna architecture has been presented. It is based on the development of 2.5 bits phase shifting elements with MEMS switches. REFERENCES [1] K. E. Peterson, Micromechanical membrane switches on silicon, IBM J. Res. Develop., vol. 23, no. 4, pp , July 1979 [2] E. R. Brown, RF-MEMS Switches for Reconfigurable Integrated Circuits, IEEE Trans. Microwave Theory Tech., Vol. 46, No. 11, pp , Nov [3] C.L. Goldsmith, Performance of low loss RF MEMS capacitive switches IEEE Microwave and Guided Wave Letters, Vol8, No. 8, August [4] J. Huang, R.J. Pogorzelski, «A Ka-band microstrip reflectarray with elements having variable rotation angle», IEEE transactions on antennas and propagation, Vol. 46, N 5, May 1998 [5] R.D. Janor, X-D Wu, K. Chang, «Design and performance of a microstrip reflectarry antenna», IEEE transactions on antennas and propagation, Vol. 43, N 9, September 1995 [6] G.H.Knittel, «Wide-angle impedance matching of phased array antennas, a survey of theory and practice», in Proceedings of the 1970 phased array antenna symposium, p [7] Phelan, H. Richard, «Spiraphase A new, low cost, lightweight phased array», Microstrip Journal, Dec 76, pp [8] J. Huang and R.J. Pogorzelski, "A Ka-Band Refllectarray With Elements Having Variable Rotation", IEEE Trans. on Antennas and Propagation, May 1998,vol 46, pp