36 research outputs found
A non-obtrusive technique to characterize dielectric charging in RF-MEMS capacitive switches
No full textDegradation and failure due to dielectric charging has been a dominant and pervasive reliability concern for RF-MEMS switches. Traditionally, the operational lifetime dictated by this degradation phenomenon is extrapolated from a series of measurements of time-dependent shifts in Capacitance-Voltage (C-V) characteristics under accelerated stress conditions. In this paper, we explain why the classical large-signal C-V methodology may lead to a pessimistic under-prediction of device lifetime. Using both simulations and experiments, we propose and verify a new small-signal characterization technique based on resonance characteristics of MEMS cantilever beams. This new technique overcomes the limitations of the classical approaches to accurately anticipate device lifetime and opens up the possibility of non-obtrusive, in-situ runtime monitoring of degradation in RF-MEMS switches. Moreover, since the technique is amenable to `parallel\u27 implementation, it has the potential to be used both as an in-line process monitor as well as to reduce the overall time to technology qualification
Charging and breakdown in amorphous dielectrics: Phenomenological modeling approach and applications
No full textAmorphous dielectrics of different thicknesses (nm to mm) are used in various applications. Low temperature processing/deposition of amorphous thin-film dielectrics often result in defect-states or electronic traps. These traps are responsible for increased leakage currents and bulk charge trapping in many associated applications. Additional defects may be generated during regular usage, leading to electrical breakdown. Increased leakage currents, charge trapping and defect generation/breakdown are important and pervasive reliability concerns in amorphous dielectrics. We first explore the issue of charge accumulation and leakage in amorphous dielectrics. Historically, charge transport in amorphous dielectrics has been presumed, depending on the dielectric thickness, to be either bulk dominated (Frenkel-Poole (FP) emission) or contact dominated (Fowler-Nordheim tunneling). We develop a comprehensive dielectric charging modeling framework which solves for the transient and steady state charge accumulation and leakage currents in an amorphous dielectric, and show that for intermediate thickness dielectrics, the conventional assumption of FP dominated current transport is incorrect, and may lead to false extraction of dielectric parameters. We propose an improved dielectric characterization methodology based on an analytical approximation of our model. Coupled with ab-initio computed defect levels, the dielectric charging model explains measured leakage currents more accurately with lesser empiricism. We study RF-MEMS capacitive switches as one of the target applications of intermediate thickness amorphous dielectrics. To achieve faster analysis and design of RF-MEMS switches in particular, and electro-mechanical actuators in general, we propose a set of fundamental scaling relationships which are independent of specific physical dimensions and material properties; the scaling relationships provide an intrinsic classification of all electro-mechanical actuators. However, RF-MEMS capacitive switches are plagued by the reliability issue of temporal shifts of actuation voltages due to dielectric charge accumulation, often resulting in failure due to membrane stiction. Using the dielectric charging model, we show that in spite of unpredictable roughness of deposited dielectrics, there are predictable shifts in actuation voltages due to dielectric charging in RF-MEMS switches. We also propose a novel non-obtrusive, non-contact, fully electronic resonance based technique to characterize charging driven actuation shifts in RF-MEMS switches which overcomes limitations in conventionally used methods. Finally, we look into the issue of defect generation and breakdown in thick polymer dielectrics. Polymer materials often face premature electrical breakdown due to high electric fields and frequencies, and exposure to ambient humidity conditions. Using a field-driven correlated defect generation model, coupled with a model for temperature rise due to dielectric heating at AC stresses, we explain measured trends in time-to-breakdown and breakdown electric fields in polymer materials. Using dielectric heating we are able to explain the observed lifetime and dielectric strength reduction with increasing dielectric thicknesses. Performing lifetime measurements after exposure to controlled humidity conditions, we find that moisture ingress into a polymer material reduces activation barriers for chain breakage and increases dielectric heating. Overall, this thesis develops a comprehensive framework of dielectric charging, leakage and degradation of insulators of different thicknesses that have broad applications in multiple technologies
Theory of charging and charge transport in “intermediate” thickness dielectrics and its implications for characterization and reliability
No full textThin film dielectrics have broad applications, and the performance degradation due to charge trapping in these thin films is an important and pervasive reliability concern. It has been presumed since the 1960s that current transport in intermediate-thickness (IT) oxides (∼10–100 nm) can be described by Frenkel-Poole (FP) conduction (originally developed for ∼mm-thick films) and algorithms based on the FP theory can be used to extract defect energy levels and charging-limited lifetime. In this paper, we review the published results to show that the presumption of FP-dominated current in IT oxides is incorrect, and therefore, the methods to extract trap-depths to predict lifetime should be revised. We generalize/adapt the bulk FP current conduction model by including additional tunneling-based current injection. Steady state characteristics are obtained by a flux balance between contacts and the IT oxide. An analytical approximation of the generalized FP model yields a steady state leakage current J ∝ exp(−B√E)(1 − C√E − D/E), where B, C, and D are material-specific constants. This reformulation provides a new algorithm for extracting defect levels to predict the corresponding charging limited device lifetime. The validity and robustness of the new algorithm are confirmed by simulations and published experimental data
Electrical breakdown in polymers for BEOL applications: Dielectric heating and humidity effects
No full textPolymer dielectrics may be used as low-k BEOL dielectrics, however, premature electrical breakdown due to high electric fields, high frequencies and ambient humidity conditions have restricted its widespread adoption. In this study, we show that dielectric heating is the primary AC degradation mechanism in polymer dielectrics, and develop an analytical model that is consistent with measured trends in stress tests under both AC and DC electric fields. We also study the effect of exposure to ambient relative humidity on the lifetime of polymer dielectrics
Universal scaling and intrinsic classification of electro-mechanical actuators
No full textActuation characteristics of electromechanical (EM) actuators have traditionally been studied for a few specific regular electrode geometries and support (anchor) configurations. The ability to predict actuation characteristics of electrodes of arbitrary geometries and complex support configurations relevant for broad range of applications in switching, displays, and varactors, however, remains an open problem. In this article, we provide four universal scaling relationships for EM actuation characteristics that depend only on the mechanical support configuration and the corresponding electrode geometries, but are independent of the specific geometrical dimensions and material properties of these actuators. These scaling relationships offer an intrinsic classification for actuation behavior of a broad range of EM actuators with vastly different electrode/support geometries. Consequently, the problem of analysis/ design of complex EM actuators is reduced to the problem of determining only five scaling parameters, which can be obtained from no more than three independent characterization experiments or numerical simulations
Strategies for dynamic soft-landing in capacitive microelectromechanical switches
No full textElectromechanical dielectric degradation associated with the hard landing of movable electrode is a technology-inhibiting reliability concern for capacitive RF-MEMS switches. In this letter, we propose two schemes for dynamic soft-landing that obviate the need for external feedback circuitry. Instead, the proposed resistive and capacitive braking schemes can reduce impact velocity significantly without compromising other performance characteristics like pull-in voltage and pull-in time. Resistive braking is achieved by inserting a resistance in series with the voltage source whereas capacitive braking requires patterning of the electrode or the dielectric. Our results have important implications to the design and optimization of reliability aware electrostatically actuated MEMS switches
Implications of Rough Dielectric Surfaces on Charging-Adjusted Actuation of RF-MEMS
No full textActuation voltage shifts due to dielectric charging is a leading reliability concern in Radio-Frequency Micro-Electro-Mechanical Systems (RF-MEMS) capacitive switches. The inability to correlate dielectric surface roughness to charge accumulation makes predictive design difficult. We apply a sophisticated dielectric charging model on representative surfaces based on Atomic Force Microscopy (AFM) data, and show that there are significant, but predictable actuation voltage shifts due to surface roughness. The results suggest that surface roughness should be considered for accurate lifetime predictions, and simple metal-insulator-metal (MIM) capacitors may serve as a useful test structure for this phenomenon
MEMS Lab Simulation Tool
No full textMEMS actuators have multiple design applications. Understanding their behavior as well as the ability to predict their actuation characteristics and voltage response is important when designing these actuators. In order to know these devices will behave, designers have to solve multiple analytical equations and experiments that can be very time consuming. Over the course of the summer a tool was created on nanoHUB that will allow users to enter information about a MEMS actuator and provide the voltage response of the actuator. To create the tool, scaling equations were first provided for various geometry configurations and the equations were next written into a function in MATLAB used for carrying out the calculations and producing curve outputs. The MATLAB code was also integrated using the RAPTURE software to create the tool on nanoHUB. The resulting tool allows users to select the geometry configuration, dimensions of the top electrode and choose from a selected list of electrodes and dielectric materials. Users also have the options of entering properties of their own materials. The final output of this tool is a plot showing the Capacitor-Voltage and the Beam displacement-Voltage sweep over the specified ranges of voltages The tool created is of interest to MEMS designers and anyone else that wants to learn about MEMS as they can study changes in either the dimensions or materials of a device and observe the response of MEMS actuators in real-time and in a modular fashion
