RF MEMS devices - IEEE Xplore

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capacitors, MEMS tunable inductors and RF MEMS mutiport switches. The tuning range of the variable capacitor was measured and found to be 280%, which far.

Proceedings of ICMENS2003: The 2003 International Conference on MEMS, NANO, and Smart Systems July 20-23, 2003 in Banff, Alberta – Canada

RF MEMS DEVICES R.R. Mansour, M. Bakri-Kassem, M. Daneshmand and N. Messiha University of Waterloo Waterloo, Ontario, Canada Abstract: This paper addresses the use of RF MEMS devices in wireless and satellite communication systems. Novel configurations are presented for MEMS variable capacitors, MEMS tunable inductors and RF MEMS mutiport switches. The tuning range of the variable capacitor was measured and found to be 280%, which far exceeds that of the traditional parallel plate MEMS variable capacitors. The MEMS tunable inductor is realized using MEMS fixed inductors, capacitors and a variable MEMS capacitor. The proposed MEMS multiport switch has demonstrated a superior RF performance up to 20 GHz. I. INTRODUCTION The MEMS technology has the potential of replacing many Radio Frequency (RF) components such as switches, inductors, capacitors, phase shifters, surface acoustic wave (SAW) devices and ceramic filters used in today’s mobile, communication and satellite systems. In many cases, such RF MEMS components would not only reduce substantially the size, weight, power consumption and component counts, but also promise superior performance in comparison with current technologies. In wireless and space applications complete RF systems, such as redundancy switch matrices, input multiplexers, and integrated front-end receivers, can be built with low cost, mass producibility and high reliability. Micromachining also enables new functionality and system capability that are not possible with current technologies. The feasibility of developing RF MEMS components with superior performance has been demonstrated in literature and is now well documented in several books published recently on the subject [1]- [3]. Over the past three years, the emphasis of RF MEMS research has been shifted to system integration, reliability and the development of new device configurations. In this paper, we present some of the novel RF MEMS devices that have been developed recently by the University of Waterloo for

RF MEMS variable capacitors, MEMS tunable inductors and RF MEMS multiport switches . II MEMS VARIABLE CAPACITORS Varactor tuning techniques are widely employed in phase shifters, oscillators and tunable filters. MEMS technology has the potential of realizing variable capacitors with a performance that is superior to varactor diodes in areas such as non-linearity and losses. A MEMS parallel plate variable capacitor with an electrostatic actuator is considered the most convenient configuration to build due to simplicity of fabrication [4]. Such type of MEMS capacitors however have a maximum theoretical tuning range of 50% due to the collapse of capacitor structure as the voltage is increased beyond the pull-in voltage [4]. A MEMS variable capacitor with a wider tuning range was proposed in [5] by making the actuation electrodes spaced differently from the capacitors plates. Such approach has yielded a 100% tuning range. While the theoretical tuning range is 100%, in practice, the capacitor should operate over a smaller tuning range away from the collapse voltage. There has been therefore a need to develop MEMS variable capacitors with a much wider tuning range. Fig. 1 illustrates a schematic diagram of our proposed MEMS capacitor. It consists of two movable plates with an insulation dielectric layer on top of the bottom plate. With the two plates being flexible, makes it possible for the two plates to attract each other and decrease the maximum distance before the pull-in voltage occurs. Moreover, the capacitor demonstrated an extended tuning range even after the two plates touched each other. The proposed capacitor is constructed using two structural layers, three sacrificial layers, and two insulating layers of Nitride. The top plate is fabricated from nickel with a thickness of 24 µm covered by a gold layer of thickness 2 µm, while the bottom plate is made of polysilicon covered by a Nitride layer of a thickness of 0.35 µm.

Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03) 0-7695-1947-4/03 $17.00 © 2003 IEEE

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Fig. 2 illustrates the different layers used to construct the capacitor using the MetalMUMPs process [6]. The 2D layers are generated using CoventorWare [7]. First, a layer of 0.5-micron oxide is deposited and patterned as illustrated in Figs 2(a)-2(b). This oxide layer outlines the area that will be used to etch a trench in the silicon substrate. The first Nitride layer of 0.35-micron thickness is deposited and pattered as illustrated in Fig 2(c). This Nitride layer forms the bottom cover of the polysilicon layer and is used as a part of the capacitor’s bottom plate. On top of the first Nitride layer, a 0.7-micron layer of polysilicon is deposited and patterned to form the bottom conductive plate of the variable capacitor as shown in Fig. 2(d). The last step in building the bottom plate of the variable capacitor is to deposit the second Nitride layer on top of the polysilicon layer to form the isolating area that prevents any electrical contact between the two plates. Thus, eliminating the sticktion problem.

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Fig. 2. The fabrication process of the MetalMUMPs that used to build the proposed variable capacitor.

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Fig. 1. A schematic diagram of the proposed capacitor

A 1.1-micron layer of second oxide is then deposited as illustrated in Fig. 2(f). The second oxide layer is etched so that the metal layer is anchored on the Nitride and a physical contact between the bottom electrode (Polysilicon) and the two outer pads is ensured. The last layer is the metal layer, which is formed of a 24 µm of Nickel with 2µm of gold on top of the Nickel layer. The last step is to etch out the sacrificial layers as well as to etch a trench in the silicon substrate. The trench etch of the substrate is determined by the first oxide layer. Once the first oxide is etched away by opening holes through the Nitride layer, the solvent will etch the isolation layer underneath. The silicon substrate is then etched to form a trench of a depth of 25µm. The total depth from the bottom plate of the variable capacitor is 27.5 µm. Fig. 3 shows a SEM picture of the proposed MEMS variable capacitor. A DC voltage sweep from 0 V up to 39 V was applied to the variable capacitor. Figure 8 illustrates the measured capacitance value for DC voltage steps over the frequency range of 1 - 1.5 GHz. At 1 GHz, the achievable tuning of the proposed capacitor is found to be 280%

Fig.3. An SEM picture of the fabricated variable capacitor.

Figure 4. Measured capacitance vs. frequency at different DC voltages.

Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03) 0-7695-1947-4/03 $17.00 © 2003 IEEE

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III MEMS TUNABLE INDUCTORS High-Q inductors find widespread use in RF transceivers circuits. Examples include filters, matching networks of LNAs and tank circuits of VCOs. The availability of tunable inductors will make it possible to easily fine tune such circuits to circumvent the manufacturing tolerances limitations. MEMS tunable inductors can be also used to construct filters with tunable bandwidth and a tunable center frequency for frequency agile applications. Variable inductors have been achieved by using a drive coil coupled to the RF inductor [8]. The variation in the mutual inductance between the two inductors yields tunability. The mutual component can be continuously increased or decreased based on the relative phase of the current in the two coils. A 100% tuning range has been achieved with this technique. However, this technique requires the use of an additional drive circuit to change the phase of the current in the drive coil. The concept of using admittance J-inverters and variable capacitors to realize tunable inductors was proposed in [9]. In this paper, however, a λ/4 transmission line was used to construct the inverter. The inverter has a limited bandwidth and would relatively occupy a large area in low frequency applications. In this paper, we propose a MEMS tunable inductor realized using lumped element inverters is presented. The lumped element admittance inverter has a wider bandwidth in comparison with a λ/4 inverter. In addition, it offers more design flexibility since some of the elements of the inverter can themselves made to be tunable. The circuit representing the tunable inductor consists of two inductors, two fixed capacitors and a shunt variable capacitor as shown in Figure 1. The tunable inductor was designed using the Multi-User MEMS Processes (MUMPs) surface polysilicon micromachining technology [10]. The MUMPs process includes three layers of polysilicon (poly0, poly1 and poly2), two layers of oxide and one layer of gold. The gold layer is deposited on the top polysilicon layer (poly2). The poly0, first oxide, poly1, second oxide, poly2 and gold have thickness of 0.5,2.0,2.0,0.75,1.5 and 0.5 µm respectively [10]. The two parallel plate variable capacitor is constructed as follows: the lower plate is made of poly1 and the upper plate is made of poly 2, then a layer of gold is deposited on the upper plate. There’s an air gap of 0.75 µm between these plates. Holes were made in the capacitor plates to ensure the etching of the oxide between these two plates. The capacitance value of 2.05pF can be achieved by using the plates of area 210 × 270 µm2. The variability of the capacitance value can be achieved by applying voltage to the variable capacitor. The top plate moves towards the fixed lower plate changing the

distance between these plates and thus varying the capacitance value. The fixed capacitors are constructed using the same concept except that no voltage source will be applied to the plates of the fixed capacitors. The area of the plates providing 1.76 pF is 200 × 280µm2. There are 8 pads used in this design: one for the DC voltage, one for ground and 6 pads for the coplanar RF input and output signals. The pads used in the design are made of a layer of poly2 and a layer of gold. Anchors are used around the edges of the pads to ensure the trapping of the oxide and protecting it from the HF etch. These pads have a significant low parasitic capacitance of 0.25 pF. The dimensions of the pads are 86 × 86 µm2. The calculation of the variable capacitor plate’s area included the value of the parasitic capacitances of these pads. The fabricated variable inductor chip is given in Fig. 5.

Fig. 5 A MEMS Tunable Inductor chip

IV. RF MEMS MULTIPORT SWITCHES The common microwave switches currently employed in the microwave industry are mechanical-type switches (coaxial & waveguide) and semiconductor-type switches (PIN diode & FET). Mechanical coaxial and waveguide switches offer the benefits of low insertion loss, large offstate isolation, and high power handling capabilities. However, they are bulky, heavy and slow. On the other hand, semiconductor switches such as PIN diodes and FET provide much faster switching speed and are smaller in size and weight, but are inferior in insertion loss, dc power consumption, isolation and power handling capabilities than their mechanical counterparts. MEMS switches promise to combine the advantageous properties of both mechanical and semiconductor switches. They offer the high RF performance and low dc power consumption of mechanical switches but with the small size, weight and low cost features of semiconductor switches.

Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03) 0-7695-1947-4/03 $17.00 © 2003 IEEE

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Over the past five years, numerous papers have been published on RF MEMS switches [1]- [3]. However, most of the research effort reported in literature has been directed toward the development of Single-Pole-SingleThrough (SPST) switches. The SPST switch is a two-port device, which acts as a simple RF relay. In today's communication systems, switches are typically used in the form of switch matrices either for redundancy or for signal routing. The use of multiport switches rather than SPST switches, as the basic building blocks, can considerably simplify the integration problem of large size switch matrices. In our knowledge, very limited work has been reported in literature on integrated multiport MEMS switches. In this paper, we present an integrated SP3T MEMS switch. Three beams with narrow-width tips are integrated on top of a co-planar transmission line. The junction where the three beams interact is inherently a wide band junction, which makes it possible to design a wideband SP3T switch with 30 dB isolation up to 20 GHz. Another feature of the proposed switch is that any of the four ports can be used as an input port to the SP3T switch. The mechanical design of the switch is analyzed using CoventorWare [7]. Fig. 6. shows the proposed SP3T MEMS switch structure. It is a compact (500 × 500 µm) coplanar series switch, consisting of three actuating beams. One end of each beam is attached to a 50 Ω coplanar transmission line, while the other end is suspended on top of another 50 Ω ҏtransmission coplanar line to form a series-type contact switch. The pull down electrodes, which are parts of the RF ground, are placed underneath the beams. The design can be implemented by integrating the beams and the substrate on one chip as the case of the SP4T switch reported in [11]. Alternatively, the SP3T switch can be implemented in a hybrid-form where the beams are micromachined separately and then integrated on an Alumina substrate using flip-chip technology. In this paper, we report the results of hybrid version of this switch. The beams are fabricated using the Multi-User MEMS Processes (MUMPs) surface micro machining [10]. Each beam is made of a Polysilicon layer of a thickness of 1.5µm covered by a gold layer of 0.5 µm thick. Release holes are accommodated for HF accessibility to the trapped oxide under the beams. The coplanar line circuit is fabricated on a 254µm thick Alumina substrate. The beams are flipped on top of a gold-coated Alumina substrate using flip chip process and gold bumps, as shown in Fig. 7. In order to improve the isolation of the switch, the beams are narrowed at the tip and the contact is performed only by small tips at the end of the beams.

With the use of flip-chip technology, a gold-to-gold contact has been realized.

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Port 1 Fig. 6. The proposed MEMS SP3T switch

The RF performance of the SP3T switch has been characterized over a wide range of frequency from DC to 40 GHz, using HFSS software. The results for the case that port 2 is in ON state and ports 3 and 4 are in OFF state are shown in Fig. 8. For the case that port 3 is ON and the others are OFF is shown in Fig. 7. It is worth mentioning that due to the symmetric configuration of the switch, the results for the case of port 4 in ON and port 2 and 3 in OFF states are similar to the case shown in Figure 7. REFERENCES [1] Hector J. D. Santos, RF MEMS Circuit Design”, Artech House, Boston, 2002. [2] V. Varadan, K. J. Vinoy and K. A. Jose,” RF MEMS and their applications”, John Wiley & Sons Ltd, England, 2002. [3] G. M. Rebeiz, “ RF MEMS, Theory, design and Technology, John Wiley & Sons Ltd, New Jersey , 2002. [4] Aleksander Dec, and Ken Suyama, “ Micromachined Electro-Mechanically Tunable Capacitors and Their Applications to RF IC’s “ IEEE Transactions on Microwave Theory and Techniques, Vol. 46, No. 12, p. 2587 December 1998. [5] Jun Zou, Chang Liu, Jose Schutt-Aine, Jinghong Chen, and Sung-Mo Kang “Development of a Wide Tuning Range MEMS Tunable Capacitor for Wireless Communication Systems” “,Electron Devices Meeting, 2000. IEDM Technical Digest. International, pp. 403 – 406, 2000. [6] A. Cowen, B. Dubley, E. Hill, M. Walters, R. Wood, S. Johnson and H. Wynands, “MetalMUMPS’s design handbook Revision 1.0,” JDS Uniphase. NC. http://www.memsrus.com/svcsmetal.html [7] www.coventor.com

Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03) 0-7695-1947-4/03 $17.00 © 2003 IEEE

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[8] D.R. Pehlke, A. Burstein and M. F. Chang, “ Extremely high-Q tunable inductor for Si-based integrated circuit applications”, IEEE, IEDM, Dig, pp.63-66, 1997. [9] J. Sinsky and C. Westgate, “Design of an Electronically Tunable Microwave Impedance Transformer”, Microwave Symposium Digest, 1997. IEEE MTT-S International, Volume: 2, 1997. [10] D. Koester, R. Mahadevan, B. Hardy and K. Markus, “MUMPs Design Handbook, Revision 7.0”. [11] G. M. Rebeiz, and J.B. Muldavin, “ RF MEMS switches and Switch Circuits”, IEEE Microwave Magazine, pp.59-71, December 2001.

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Fig. 8. Simulated results of the MEMS 3PST switch.

(d) Fig. 7 : The fabrication process of the SP3T switch. a) the PolyMUMPs chip, which has the beams, b) The Alumina substrate, c) The PolyMumps chip is attached to the substrate, d) the SP3T switch after realizing the beams from the PolyMUMPs chip.

Proceedings of the International Conference on MEMS, NANO and Smart Systems (ICMENS’03) 0-7695-1947-4/03 $17.00 © 2003 IEEE

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