Reliability of RF MEMS Switches at High and Low Temperatures: Modeling, Simulation and Experiment

This paper examines the reliability of RF MEMS switches when operational temperatures in the range -60C to 100C are envisioned. The basic operation of a capacitive MEMS switch is described and two tools to examine device reliability, modeling and on-chip experimentation, are discussed for the case of capacitive MEMS switches. 1-D, 2-D and 3-D models are presented with emphasis on 3-D coupled-field finite element analysis (FEA) including temperature effects. Results and findings from the 3-D simulations are reported. In particular, the advantages of employing corrugated membranes in the design of RF MEMS switches are assessed. Their performance in terms of reliability as a function of temperature is quantified. For assessing reliability experimentally, the membrane deflection experiment (MDE) is reviewed due to their on-chip characteristic. How this combined experimental/computational methodology is used for identifying material properties and device performance is also highlighted. Introduction Reliability of MEMS switches is of major concern for long-term and broad applications and it is currently an area of intense research [1]. For capacitive MEMS switches, the major reliability problem is stiction between the metal layer (top electrode) and the dielectric layer covering the bottom electrode. The failure is believed to be result of charge build-up in the dielectric material. This charge build-up is strictly related to the actuation voltage [2]. A reduction in the actuation voltage by 6 V results in a 10-fold increase in the lifetime of typical MEMS switches. Electromechanical analysis and on-chip experimentation are vital for improved design of these devices. Some applications of RF MEMS switches, such as aircraft condition monitoring and distributed satellite communication, require low operational temperature (e.g. -60C). By contrast, the temperature during device packaging can reach 200C and as a result affect the post-package device performance. Hence, device reliability as a function of temperature presents a new challenge for the designer of MEMS switches. It has been reported that a moderate temperature increase may cause buckling of the switch structure, which leads to a premature failure of the device. On the other hand, temperature reduction could result in a significant increase of the pull-in voltage, which is undesirable [3]. Therefore, it is relevant to design a switch with materials and structure almost insensitivity to temperature variations. Investigation of the reliability of capacitive MEMS switches at high and low temperatures is the subject of this paper. Electromechanical models, accounting for thermally induced stresses, and experimental techniques suitable to this endeavor are discussed with emphasis on the usage of corrugated membranes as a candidate for stress relaxation within the structure. The paper is organized as follows: 1) various models for electromechanical analysis of MEMS switches are presented and the applicability of each model is discussed; 2) corrugated membranes are examined in detail in the context of reliability versus temperature using 3D coupled-field Finite Element Analysis (FEA); 3) membrane deflection experiment (MDE) technique is reviewed for the purpose of reliability testing at various temperatures; 4) concluding remarks on the design of RF MEMS switches are summarized. Electromechanical Modeling of RF switch A typical capacitive RF MEMS switch consists of a fixed-fixed thin metallic membrane suspended over a bottom electrode insulated by a dielectric film. When the switch is not actuated, there is a low capacitance between the membrane and the bottom electrode, and the device is in the OFF state [4]. When a voltage is applied between the movable structure and the fixed bottom electrode, electrostatic charges are induced on both the movable structure and the bottom electrode. The electrostatic charges cause a distributed electrostatic force, which deforms the movable structure. In turn, such deformation leads to storage of elastic energy, which tries to restore the structure to its original shape. The structure deformation also results in the reorganization of all surface charges on the device. This reorganization of charges causes further structural deformation; hence, the device exhibits a highly nonlinear coupled electromechanical behavior. Until a certain applied voltage, so-called pull-in voltage or actuation voltage, an equilibrium position exists through a balance between the elastic restoring force and electrostatic force. After pull-in, the device is in the ON state. In order to accurately describe the switch deformation and predict the pull-in voltage, an effort on modeling has to be pursued. In the following section, we will discuss a simple 1-D parallel-plate actuator model [5], a 2-D distributed model [6], and a 3-D fully coupled model [7-8]. The analyses and simulations are dedicated to capacitive MEMS switches though they are also applicable to other types of electrostatic devices. A. 1-D Parallel-Plate Actuator Model A first-order approximation of this device is a system consisting of two parallel plates separated by gap 0 g , with one plate fixed on the substrate and the other suspended by a linear spring (Figure 1(a)). When a voltage V is applied across the plates, the force balance equation is given by, 0 ) ( 2 2 0 2 = − − w K w g AV eff ε (1) where L T L Ebt Keff 8 32 3 3 + = is the effective structural stiffness, bt T ) 1 ( 0 ν σ − = , A the area of the movable plate, ε the electric permittivity of free space, w the deflection, E the Young’s modulus, t the plate thickness, 0 σ the biaxial residual stress presumed present before the plate shape is etched, and ν the Poisson’s ratio. As V increases, the movable plate is deflected down gradually. However, at the pull-in voltage, 3 0 27 8 g A k V eff in pull ε = − , the system becomes unstable and the plate suddenly collapses. The corresponding gap is 0 3 / 2 g . This linear spring approximation works quite well for the small deflection regime of the switch membrane. However, for a capacitive MEMS switches the deflection is usually tens of times larger than its thickness [4]. Hence, to describe the switch membrane deflection, a Duffing spring with a nonlinear term can be used instead. Then the force balance equation becomes 0 ) ( 2 3 3 1 2 0 2 = − − − w k w k w g AV ε (2)