RSM2013 Proc. 2013, Langkawi, Malaysia

Design and Analysis of a Low-Voltage Electrostatic Actuated RF CMOS-MEMS Switch

Ma Li Ya#1, Anis Nurashikin Nordin*2, Norhayati Soin#3 #Department of Electrical Engineering, University of Malaya

50603 Kuala Lumpur, Malaysia *Electrical and Computer Engineering Department, International Islamic University Malaysia

53100 Kuala Lumpur, Malaysia 1maliya8445@gmail.com, 2anisnn@iium.edu.my, 3norhayatisoin@um.edu.my

Abstract— This paper presents the design and analysis of a radio frequency (RF) micro-electromechanical system (MEMS) switch with low actuation voltage using MIMOS 0.35μm complementary metal oxide semiconductor (CMOS) process. The advantage of this RF MEMS switch is very low actuation voltage design which is compatible with other CMOS circuit without employing a separate on-chip voltage source or charge pump unit. Moreover, using CMOS technology to design can highly simplify the fabrication process, reduce the cost and improve the device performance. The RF MEMS switch is a capacitive shunt-connection type device which uses four folded beams to support a big membrane above the signal transmission line. The pull-in voltage, von Mises stress distribution and vertical displacement of the membrane, up-state and down-state capacitances, as well as the switch impedance is calculated and analyzed by finite element modelling (FEM) simulation. Keywords— RF, switch, low-voltage, CMOS-MEMS

I. INTRODUCTION A radio frequency (RF) micro-electro-mechanical system

(MEMS) switch, as an essential device in RF applications, has attracted a lot of attentions during last twenty years. RF MEMS switches are devices that use mechanical movement to achieve a short or open circuit in an RF transmission line; therefore, they are normally integrated together with a planar transmission line or coplanar waveguides (CPW) transmission lines. There are, at least, three parts consisted in a RF MEMS switch, namely, a metal beam, a transmission line, and a lower pad which can be part of the transmission line. Electrostatic shunt capacitive switches, as one type of the most popular topologies in RF MEMS switches, have very wide operation-frequency range and been designed into many different ways, as shown in [1-5]. All of these designs have similar working principle. Basically, when a bias voltage is applied between the metal beam and a lower pad, a coupling capacitance exists; the switch is turned on and the signal line becomes an open circuit. Otherwise, no bias voltage supplied, no coupling capacitance occurring, the switch is turned off and the signal line is a short circuit while RF signal can be propagated through the transmission line.

Comparing with other kinds of RF switches, such as field effect transistor (FET) switches and PIN diodes, RF MEMS switches have many significant advantages, such as negligible power consumption, less series capacitance and resistance,

very high cut-off frequency, less loss, and good isolation. With the development of complementary metal-oxide-semiconductor (CMOS) technology and very large scale integration (VLSI), RF MEMS switches can be manufactured together with CMOS circuit on the same chip using relative low cost and getting a better cut-off frequency range than any other method. In another words, employing CMOS-MEMS technology for RF components can reduce cost, increase performance, and improve yield and accuracy [6]. However a big challenge for electrostatic actuated RF MEMS switch is high pull-in (or actuation) voltage which necessitates the presence of a separate on-chip voltage source or charge pump to operate the switch [7]. Many researchers have tried to propose diverse RF MEMS switch designs to either reduce actuation voltages or employ standard CMOS technologies, or consider the both aspects. For example in [8], a bi-stable RF MEMS switch was designed with a low pull-in voltage of 5V where a cantilever beam with latching properties was used, but not in CMOS technology. In 2010, S. Fouladi et al. [9] have designed a capacitive RF MEMS switch fabricated in standard 0.35μm CMOS technology, which has an insertion loss less than 1.41dB, return loss better than 19dB, and isolation of more than 19dB all over the frequency band from 10 to 20 GHz. However, an actuation of 82V on the actuation electrodes and 25V applied to the RF port through a bias-tee need to be connected to the ports of the network analyzer [9]. In [10], three different types of beams were employed in RF MEMS switches design by CMOS technology. The results show that the stiffness of beams in type-a, type-b and type-c switch are 1, 0.63 and 0.27N/m, respectively, which lead to their pull-in voltages are 21V, 13V and 7V correspondingly. So far, there are few robust design of RF MEMS switches not only using CMOS technology but also with very low actuation voltages.

In this paper, a RF CMOS-MEMS switch using shunt capacitive topology and electrostatic actuation is proposed, which only needs a 3V pull-in voltage to be well compatible with normal voltage source in CMOS circuits. The rest of the paper is divided into the following sections: section 2 presents the detail designs of RF switch according to the design objectives and specifications. Section 3 displays the simulation results of applied voltage with the membrane displacement, capacitances, stress distributions and the switch characteristic impedance. The last part is the conclusion.

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RSM2013 Proc. 2013, Langkawi, Malaysia

II. RF CMOS-MEMS SWITCH DESIGN RF MEMS switches are important components in wireless

communication systems. This section outlines a novel design of the RF shunt capacitive switch, its design specifications, as well as its physical dimensions which are derived from the pre-set specifications.

A. Switch Model Design The structure of the RF MEMS switch is illustrated in Fig.

1(a). The RF MEMS switch consists of a membrane, four folded beams, anchors and CPW transmission lines which involves two ground lines (G) at two sides and one signal line (S) in the middle. The anchors and the ground lines are connected together in order to transmit the signal to the ground, when the switch is actuated. Fig. 1(b) shows the cross-section view of the switch. On the top surface of the signal line, there is a very thin dielectric layer to form a coupling capacitor between the signal line and the membrane and to prevent them connected together. Fig. 1(c) displays the geometric parameters of the membrane and beams, where the holes are used to easily release the membrane. Their small dimensions guarantee the up-state capacitance not to be affected since the fringing fields [11].

When a bias voltage is applied between a membrane and the signal line, an electrostatic force is induced on the membrane. In the beginning, this electrostatic force equals to the restoring force which is basically generated in the four beams. With the applied voltage increasing, when the membrane is at the height of (2/3)g0, the increase in the electrostatic force is greater than the increase in the restoring force, resulting in i) the beam position becoming unstable and ii) collapse of the beam to the down-state position [11]. Therefore, the applied voltage which pushes the membrane at the position of (2/3)g0, can be approximately calculated as the pull-in voltage of the switch, which is illustrated in (1).

A

kgVVg

p0

30

32 27

80

(1)

Where, k is the spring constant of the membrane; g0 is the initial gap between the membrane and the signal line; ɛ0 is the permittivity of air, 8.854×10-12F/m; and A is the area of the membrane, namely the product of the membrane’s width and length, W×L.

B. Design Specifications MIMOS 0.35μm CMOS process is employed in this RF

MEMS switch design; and double poly layers and three metal layers are used in this process. In order to get the largest capacitance ratio, which can guarantee a big isolation for switch-off state and small insertion loss for switch-on state, Metal 1 and Metal 3 were chosen as the CPW lines and membrane respectively; Metal 2 was removed as air gap. Table I lists the thickness parameters according to 0.35μm CMOS process.

The objective of this work is to design a RF MEMS

capacitive switch with 3V actuation voltage in order to be compatible with normal CMOS circuits’ supply, and a 3GHz operating frequency to work for telecommunication systems. From (1) it can be seen that a low pull-in voltage capacitive switch should have a small spring constant and big membrane design according to the model of Fig. 1(a). The larger membrane area induces a larger up-state capacitance as shown in (2). Considering the relative small gap between two electrodes and following the most up-state

TABLE I EACH LAYER THICKNESS

Name Membrane Dielectric layer CPW lines Air gap

Thickness 0.8 μm 0.1 μm 0.624 μm 2.2 μm

(a) Overall view

(b) Cross-section view

(c) Geometric parameters

Fig. 1 RF MEMS switch design

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RSM2013 Proc. 2013, Langkawi, Malaysia

capacitances’ design as shown in [11], 0.1pF was assumed first. And the area of membrane (A) can be calculated by (2) which is around 26000μm2 with ɛr of 4.5. Then the relationship of pull-in voltage and spring constant can be plotted as presented in Fig. 2, which clearly shows that with lower pull-in voltages, small spring constant beams need to be designed. Using solid mechanical simulation in Comsol Multiphysics 4.3® and the optimum calculations with L1, L2 and L3 which are labelled in Fig. 1(c), all the parameters are obtained and listed in Table II.

r

du t

g

AC

0

0 (2)

Where, Cu is the up-state capacitance; td is the dielectric thickness; and ɛr is the permittivity of dielectric material, in this CMOS technology, which is 4.5.

III. SIMULATION RESULTS OF THE RF SWITCH The finite element method software, Comsol Multiphysics

4.3®, is utilized to simulate the behaviours of the RF CMOS-MEMS switch. Its model is established in accordance with the dimensions in Table I and II. The material of signal line, membrane, and four beams is set as aluminium with Density of 2700kg/m3, Young’s modulus of 70GPa and Poisson’s ratio of 0.33 which directly follows the setting from the software material library. The boundary conditions are set as i) all of membrane, beams and signal line are assigned as Linear Elastic Dielectric materials first which has the function of computing the model in both electric domain and mechanical domain; ii) the signal line and four beams’ ends are set as fixed part; and iii) potential of the signal line is appointed to zero to simplify the computer simulation; and the membrane is assigned as a terminal in order to supply an

actuation voltage. Then the model is meshed using normal size free-tetrahedral elements. Finally, with an applied voltage on the membrane, vertical displacement, capacitance and stress distributions of the switch are performed and evaluated as followings.

Fig. 3 illustrates the relationship among the pull-in voltage, the membrane displacement and the capacitance induced in the switch. It obviously shows that i) before the pull-in voltage applied to the switch, the membrane has no displacement, and the up-state capacitance is 0.1pF as designed in section 2-B. ii) When the applied voltage increases but less than the pull-in voltage, the membrane has slight displacement and up-state capacitance almost no much change. iii) Once the membrane at the position of (2/3)g0 which is around 0.7μm in this design, only increasing a bit of the applied voltage, then the membrane directly collapses on the lower electrode and get a down-state capacitance which is around 10.36pF. The simulated results show that the switch has a pull-in voltage of 3V and capacitance ratio of about 100.

Fig. 4 presents the displacement distribution of the switch during the actuated state. In this model, with four folded beams supporting, the membrane almost exhibits a uniform out-of-plane displacement. Fig. 5 displays the stress distribution of the membrane, which shows that the maximum von Mises stress is located at the end of each folded beams and the value of 22.765MPa is far less than the aluminium’s yield strength about 90MPa [1], which makes sure the motion of the switch can be operated in the elastic range.

The characteristic impedance of the CPW lines in the RF MEMS switch is calculated by the Agilent AppCAD tool. The thickness of each layer and the dielectric material are following the 0.35μm CMOS process. The width and length of the signal line is taken from Table II. With operation frequency of 3GHz, in order to design 50Ω impedance, the gap (G) between signal line and ground line was set as 3.9μm. Fig. 6 demonstrates the parameters’ setting and calculated result of the CPW characteristic impedance, which should proper matches with the impedance of 50Ω in the network analyzer.

Fig. 3 Applied voltages versus membrane displacement and capacitance

Fig. 2 Relationship of designed spring constant and pull-in voltage

TABLE II

MEMBRANE AND BEAM DIMENSIONS

Parameter W L w L1 L2 L3 Dimension (μm) 80 325 18 35 90 80

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RSM2013 Proc. 2013, Langkawi, Malaysia

Fig. 4 Displacement distribution of the RF switch

Fig. 5 Stress distribution of the RF switch

Fig. 6 Simulation of the CPW line characteristic impedance

IV. CONCLUSIONS A simple structure of RF MEMS switch using MIMOS

0.35μm CMOS process has been well designed and simulated. The RF switch employs with four folded beams and designs into a shunt-capacitive topology. A very low pull-in voltage of 3V can be used to actuate the RF switch and compatible with the most CMOS circuit supply source, which is the most advantage of this work. Moreover, using CMOS technology to design the model can highly simplify the later fabrication steps and cost. The mechanical properties of the membrane’s displacement and stress distribution, as well as the electrical properties of the applied voltage and capacitance have been simulated and the values are all in good agreement with the design objectives. The characteristic impedance of CPW line is 50.2Ω by simulation. The capacitance ratio of the RF MEMS switch is around 100.

ACKNOWLEDGMENT The authors would like to thank the financial support by the

RACE fund (RACE 12-006-0006).

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