Roberto Gaddi

Experimental Validation of Mixed Electromechanical and Electromagnetic Modeling of RF-MEMS Devices Within a Standard IC Simulation Environment

The validity and applicability of a high-level simulation approach of radio-frequency microelectromechanical-system (RF-MEMS) devices, based on a library of analytical compact models of elementary MEMS components, are investigated through an extensive comparison between simulation results and measurements of some representative devices (variable capacitors and series ohmic switches). The in-house developed simulation tool is implemented in a standard IC simulation environment supporting behavioral description capabilities. The devices are built in a silicon substrate technology with suspended gold membranes. We analyze the mechanical, electrical, and RF response of the devices. The RF behavior is modeled by extracting a lumped element network from measured S-parameters (scattering-parameters) to account for parasitic effects and by wrapping this network around the intrinsic MEMS device simulated with the compact models. We show that an accuracy within 5% is obtained in all considered physical domains and conditions, provided that some effective parameters (including the residual air gap in the actuated state and the RF parasitic elements) are properly extracted from measurements and accounted for in the simulations. The main factors limiting the models predictive capability are due to process nonidealities, such as plate bending due to residual stress gradient, oxide charging, surface roughness, and suspended membrane thickness variations, rather than for instance in-plane geometric process variations.

MEMS technology integrated in the CMOS back end

The Nanomech™ MEMS technology platform which implements arrays of MEMS devices embedded inside the CMOS back end is described. The key advantages of this integration approach are described in terms of achieving a reliable MEMS technology process within competitive cost requirements, without any need for dedicated process, material and packaging development. The choice of materials is also providing a strong and reliable technology for harsh environment applications where cost is also a key. Data is shown providing a broad picture of the Nanomech™ technology and its potentials for applications.

Reliability Issues in RF-MEMS Switches Submitted to Cycling and ESD Test

RF-MEMS switches have potential prerogatives better than traditional solid state devices, but the presence of mechanical movement introduces new classes of reliability issues that are not found in traditional devices. In this work we have carried out an extensive electrical characterization in order to identify the dynamic response of RF-MEMS switches driven in different conditions of voltage and actuation time. Furthermore, probably due to their recent introduction to the market, the robustness of MEMS submitted to ESD stresses has been also poorly investigated. We have studied, for the first time to our knowledge, the effects of TLP-ESD events on RF-MEMS switches identifying a very critical ESD sensitivity

Reconfigurable MEMS-enabled LC-tank for multi-band CMOS oscillator

A wide frequency span reconfigurable MEMS-enabled LC-tank network for a multi-band RF oscillator is implemented, made of ohmic RF-MEMS switches, spiral suspended inductors and metal-insulator-metal (MIM) capacitors all integrated on a high resistivity silicon MEMS substrate. The LC-tank is connected to a differential cross-coupled n-MOS pair on a separate CMOS substrate, by chip-on-chip integration and wire bonding, leading to the complete MEMS-CMOS oscillator prototype. The separate LC-tank and the complete oscillator are both tested on-wafer through RF-probing. Measurement results show two resonance frequencies of 1.375 GHz (Q = 7.2) and 3.605 GHz (Q = 14.4) for the MEMS-enabled LC-tank. The MEMS-CMOS oscillator shows a phase noise of -124.5 dBc at 1MHz frequency offset, with 3mA bias current at 1.2 V power supply. Overall oscillator performances well compare with state-of-the-art non-reconfigurable CMOS oscillator designs.

Non Linear Compact Modeling of RE-MEMS Switches by Means of Model Order Reduction

In this work, a new method is presented for the extraction of low order electromechanical models for dynamic nonlinear simulation of radio frequency (RF) microelectromechanical (MEMS) switches, using model order reduction (MOR). The method is based on a separate modeling of the electrical and the mechanical part of the problem, which are coupled at circuit level. The derived reduced order model includes effects of initial stress in the device materials and enables a substantial speed-up of large signal dynamical analysis. The accuracy of the model has been demonstrated by comparing the simulated and measured dynamic response of a fabricated device.

RF-MEMS wafer-level packaging using through-wafer via technology

This paper presents wafer-level packaging (WLP) solution for RF-MEMS applications based on through-wafer via (TWV) technology in high-resistivity silicon (HRS). A pre-processed HRS capping wafer containing recesses and vertical Cu-plated TWV interconnect is, after alignment, bonded to the RF-MEMS wafer providing environmental protection and easy signal access. Optionally, cavities can be formed simultaneously with TWV in the capping wafer, which allows hybrid co-integration of additional IC dies while maintaining overall thickness of the resulting SMT compatible package. This cavity can also be used for a first-level wafer-to-wafer alignment accuracy check. After bonding, the s-parameter measurement at giga hertz level shows little influence introduced by the capping substrate.

Non-linear electromechanical RF model of a MEMS varactor based on VerilogA© and lumped-element parasitic network

In this paper we discuss an approach for the implementation of non-linear electromechanical and RF models of complete RF-MEMS devices within a standard circuit simulator, such as Cadence Spectrecopy or Agilent ADStrade. The intrinsic electromechanical core is based on a structural hierarchical model library implemented in VerilogAcopy language. Moreover, a surrounding lumped element network accounting for the electromagnetic RF behaviour of the MEMS device is extracted from S-parameters measured data. The whole nonlinear model network, composed of the intrinsic VerilogAcopy core plus lumped-element parasitics, can be analysed within a radiofrequency circuit simulator schematic. The model is fully validated against electromechanical and RF static and dynamic measurements, showing good agreement between simulations and experimental results. The presented modeling approach can be the basis for the development of complete RF-MEMS design technology libraries, aiming at optimum hierarchical designs of MEMS-enabled RF circuit blocks.

Mixed-Domain Simulation of Intermodulation Distortion in RF-MEMS Capacitive Shunt Switches

A hierarchical MEMS design library, implemented in SpectreHDL® within the Cadence® simulation framework, is applied to the study of distortion phenomen in RF-MEMS capacitive shunt switches. Two-tone mixeddomain transient simulation of the non-actuated switch, operated at high-power, directly shows, for the first time, self-biasing modulation at the tones spacing frequency. Adjacent-channel intermodulation products are observed and quantified, proving that the adopted mixed-domain simulation approach is the key tool for addressing system-level specifications in RF-MEMS design for wireless applications.

Radiated efficiency of aperture tuned antennas: Comparison of MEMS Digital Variable Capacitor and fixed ceramic capacitors

Aperture tuned antennas are becoming more common in Smart Phone Handsets to address radiated efficiency issues caused by industrial design constraints and increased frequency requirements. This paper focuses on a real application example of a MEMS Digital Variable Capacitor (DVC) from Cavendish Kinetics in antenna aperture tuning. The OTA performance is bench-marked against using state-of-the-art fixed-valued discrete capacitors in place of the DVC, showing similar radiation efficiency between the two. These results directly show how the low loss and low parasitics of the MEMS DVC can enable antenna aperture tuning applications.

The hierarchical HDL-based design of an integrated MEMS-CMOS oscillator

A hierarchical and mixed-domain design approach is implemented within the Cadence/sup /spl reg// simulation framework and applied to an integrated MEMS-CMOS oscillator. Parameters concerning both the electronic circuit description at transistor level and the MEMS structure defined by its topology and dimensions, are available during simulation. This approach can lead to a full optimisation of the complete MEMS-CMOS sub-system, with a view to both electrical and mechanical specifications.