Research paperDesign and characterization of RF MEMS capacitive shunt switch for X, Ku, K and Ka band applications
Graphical abstract
Introduction
Micro-Electro-Mechanical-System (MEMS) consists of sub-micron mechanical and electronic components on a single module [1]. In the last two decades, various RF MEMS switches have been developed using state of the art CMOS fabrication process for communication system [2]. At present, RF MEMS switch has the potential to replace the solid state switch and conventional electromechanical relay, due to its better RF performance such as high isolation, low insertion loss, high linearity, negligible DC power consumption and low intermodulation products as compared to other counterparts [[1], [2], [3], [4]]. Practically, RF MEMS switches are being widely used in numerous applications including transmit/receive (T/R) modules, like reconfigurable antenna, RF phase shifter, phase-array radar, filter, mixer, and switching networks [1]. MEMS based switch can be actuated by four different types of actuation mechanisms such as electrostatic, piezoelectric, magentostatic, and thermal actuation [1]. However, electrostatic actuation is widely used due to its compatibility with IC technology, small electrode size, negligible DC power consumption, and short switching time [1]. Depending upon their contact, broadly RF MEMS switches can be classified as DC-series and capacitive-shunt switches. DC-series switch utilizes metal-to-metal contact, and can be realized in cantilever and fixed-fixed beams configurations [5]. In up-state these switches act as electrically open circuit whereas an electrical connection is made for RF power flow in down state position [1]. On the other hand, shunt capacitive switches are typically configured in fixed-fixed configuration, wherein RF power flows in upstate from one port to another and in down state total input RF power flows to ground through top metal beam/bridge electrode [6].
RF MEMS capacitive shunt switches typically use a thin dielectric layer of silicon nitride (Si3N4, εr = 7.5) [6], between top and bottom electrodes that provide isolation between them. Other dielectric material having high dielectric constant such as hafnium dioxide (HfO2, εr = 25) [6], aluminum nitride (AIN, εr = 9.8) [7], piezoelectric lead zirconatetitanate (PZT, εr = 190) [8], strontium titanate oxide (εr = 120) [9] can also be used in place of silicon nitride layer as desired by the application. A capacitive shunt switch acts as a two state digital capacitor: exhibiting small capacitance Cup (fF) in up position and large capacitance Cd (pF) in down state of the switch. The ratio of Cd/Cup is figure of merit (FOM) for these switches [1].
The performance of the RF MEMS switches is mainly limited by reliability issues due to its movable mechanical structure and is a serious concern for long term applications. Metal-to-metal contact switches generally suffers from stiction problem. Stiction occurs when surface adhesion forces are greater than resorting force of the structure [10,11].It can be the improved by maximizing beam restoring force, while keeping the actuation voltage below 60Vand given an anti-stiction coating of siloxane self-assembled monolayers on the oxide [12,13]. While in case of capacitive switches reliability is mostly limited by dielectric charging. At high electric field across thin dielectric layer, charges get trapped affecting C-V characteristics of the devices. Dielectric charging can be minimized by various methods like, by using different dielectric materials and bipolar actuation [[14], [15], [16], [17], [18]].
Many researchers have reported RF MEMS capacitive shunt switches with various topologies and varied RF and electromechanical characteristics. Goldsmith et al. [19] fabricated a capacitive switch with insertion loss (< 0.25 dB at 35 GHz) and isolation (35 dB at 35 GHz) with membrane of aluminum alloy. Fouladi et al. [20] developed a capacitive switch on silicon using 0.35 μm CMOS technology, wherein an insertion loss <0.98 dB, return loss <13 dB and isolation from 12.4 to 17.9 dB were measured. Furthermore, Badia et al. [7], studied the effect of AlN in place of silicon nitride; wherein 12 dB more isolation was measured with AlN as compared to silicon nitride. Deng et al. [21], reported RF switch using a surface micromachining process with pull-in voltage and isolation of 16 V &19 dB respectively. Ansari et.al [22] reported a return loss of 11.5 dB, 0.5 dB insertion loss over a frequency band of 1–40 GHz using SiO2 dielectric layer.
Despite previous studies, very few MEMS based switches exhibit better isolation and insertion loss over a wide range of frequencies varying from 0 to 40 GHz. Hence, in this work we present the detailed design, fabrication and characterization of RF-MEMS capacitive shunt switch demonstrating an isolation and insertion loss better than 20 dB and 0.35 dB respectively for X (8–12 GHz), Ku (12–18 GHz), K (18–26 GHz) and Ka (26–40 GHz) band applications. The switch prototype is developed on silicon substrate (εr = 11.8, tanδ = 0.01 with high resistivity ρ > 8 kΩ-cm). Further, the experimental results are validated by performing equivalent circuit analysis implementing method of moment (MoM) based technique. In addition, we have evaluated the critical stress, switching time, and hysteresis based on the finite element method (FEM). System on chip (SoC) concept can be implemented using silicon as a substrate material for this MEMS switch. Utilizing state of the art CMOS process, other RF components can be integrated along with MEMS switch monolithically, providing compact, high performance and cost effective SoCs for space applications.
Section snippets
Switch design
High resistive silicon (ρ > 8kΩ-cm, tanδ = 0.01 and εr = 11.8) of 675 ± 20 μm thickness has been considered here. The 3D structure of the capacitive shunt switch designed in Coventorware is shown in Fig. 1(a).The cross sectional view of high resistive silicon switch using fixed-fixed beam in up and down states are given in Fig. 1(b) and (c).To minimize the losses and dispersions in the substrate, a silicon dioxide buffer layer (100 nm) is introduced, over which coplanar wave guide (CPW)
Fabrication process
Fabrication process flow of this MEMS switch is described in Fig. 5. After conventional cleaning step, 1 μm silicon dioxide was globally deposited on 6″ high resistive (> 8 kΩ-cm) silicon substrate. This insulator layer acts as buffer layer between silicon substrate and top metal lines. In the next step, 1 μm aluminum metal alloy was DC sputtered and patterned to form bottom metal actuation electrodes and CPW line. Further, to form capacitive contact and to isolate top metal beam from actuation
DC characterization
We have utilized Agilent's Semiconductor Device Analyzer (B 1500) for the CV characterization of the considered switches. The measurement setup is installed inside the clean room facility (Class 1000) with controlled ambient conditions (i.e. temperature: 20–22 °C, atmospheric pressure (760 Torr), and humidity: 40–45%). The capacitance is measured as function of voltage varied from 0 to +24 V with a voltage step of 2 V at the frequency of 1 MHz. Fig. 6(a) demonstrates the top view of the
Conclusion
In this work, we have carried out the design, fabrication and characterization of a capacitive type shunt switch having fixed-fixed beam configuration catering X (8–12 GHz), Ku (12–18 GHz), K (18–26 GHz) and Ka (26–40 GHz) band applications. The typical process flow has been outlined here considering CMOS –compatible foundry specifications. The DC and RF-characterization of the MEMS switch have been carried out on a realized prototype structure. The device has demonstrated a pull-in voltage of
Declaration of Competing Interest
None
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