Xavier Rottenberg

Analytical Model of the DC Actuation of Electrostatic MEMS Devices With Distributed Dielectric Charging and Nonplanar Electrodes

This paper gives a new insight into the problem of the irreversible stiction of RF microelectromechanical systems (MEMS) attributed to the dielectric charging. We present a model for the electrostatic actuation of MEMS devices taking into account the nonuniform distributions of the air gap and the charges in the dielectric layer. The major result of our study is the impossibility to invoke the sole uniform dielectric charging phenomenon to explain the irreversible stiction of electrostatic MEMS devices. In the absence of other forces, a nonzero variance of the charge distribution is required to explain the stiction of the device. Considering only uniform residual charge densities, previous reported works could only account for a drift of the actuation characteristics as a whole. In case of a uniform air-gap distribution, our analytical model can already account for an increase of the up-capacitance, a shift of the - , its narrowing, and the stiction by a closure of the pull-out window. We further show that the combined nonuniformities of air gaps and charges break the symmetry of the actuation characteristics. The asymmetry can be such that one of the pull-in points disappears, which is replaced by a continuous tuning range while the other pull-in point still exists.

Low-Loss Singlemode PECVD Silicon Nitride Photonic Wire Waveguides for 532–900 nm Wavelength Window Fabricated Within a CMOS Pilot Line

PECVD silicon nitride photonic wire waveguides have been fabricated in a CMOS pilot line. Both clad and unclad single mode wire waveguides were measured at λ = 532, 780, and 900 nm, respectively. The dependence of loss on wire width, wavelength, and cladding is discussed in detail. Cladded multimode and singlemode waveguides show a loss well below 1 dB/cm in the 532-900 nm wavelength range. For singlemode unclad waveguides, losses 1 dB/cm were achieved at λ = 900 nm, whereas losses were measured in the range of 1-3 dB/cm for λ = 780 and 532 nm, respectively.

Wafer-level packaged RF-MEMS switches fabricated in a CMOS fab

Reports on wafer-level packaged RF-MEMS switches fabricated in a commercial CMOS fab. Switch fabrication is based on a metal surface micromachining process. A novel wafer-level packaging scheme is developed, whereby the switches are housed in on-chip sealed cavities using benzocyclobutene (BCB) as the bonding and sealing material. Measurements show that the influence of the wafer-level package on the RF performance can be made very small.

Optimization of 0-level packaging for RF-MEMS devices

This paper reports on the optimization of the 0-level package for RF-MEMS devices like switches and tunable capacitors. The 0-level package consists of an on-chip cavity obtained by flip-chip mounting a capping chip over the RF-MEMS device, using BCB as the bonding and sealing material. A process for realizing low-profile packages, with caps less than 100 /spl mu/m thick, is described. Coplanar RF feedthroughs are implemented using BCB as the dielectric. It is experimentally shown that a 0-level package using capping chips made of low-loss high-resistivity materials and having a cavity height larger than about 45 /spl mu/m, has a negligible impact on the microwave characteristics of an RF-MEMS device, built on a 50 /spl Omega/ CPW line with ground-to-ground spacing of 150 /spl mu/m.

An electrostatic fringing-field actuator (EFFA): application towards a low-complexity thin-film RF-MEMS technology

This paper presents a novel electrostatic actuator using fringing fields as the actuation mechanism, i.e. an electrostatic fringing-field actuator or EFFA. The novel device is produced on an insulating substrate in a simple two-mask process involving only one sacrificial layer and one metallization. To demonstrate the EFFA capabilities, we produced and characterized EFFAs in various technological implementations of remarkable simplicity. This simplicity allows a vast flexibility for the processing and as a result strongly eases the integration of the EFFAs into existing technologies. For the basic device, we report a non-de-embedded measured capacitance ratio of 1:3 and a lifetime of more than 107 cycles with 40 V bipolar actuation at 100 Hz in N2 atmosphere. Both the capacitance ratio and the C–V profile were tuned by modifying the technology, e.g. coating the substrate before processing the EFFAs and the design, e.g. switching from clamped-free to clamped–clamped devices. We finally report a complete RF-MEMS technology using the EFFAs as single actuators. Switchable and tunable LC tanks, phase shifters, series and shunt parallel-plate capacitors actuated as relays and capacitive relays are presented to demonstrate the possibilities of this technology.

Evaluation of platinum as a structural thin film material for RF-MEMS devices

Surface micromachined metal armatures are commonly used for MEMS applications of which RF-MEMS is the most well known. In most cases metals with a high conductivity, such as aluminum or gold, are used. These metals often have a low melting point and therefore have a low thermal stability and show plastic deformation of the structures at relatively low temperatures (<200 °C). High melting point metals, such as platinum, are expected to show plastic deformation only at higher temperatures which makes them interesting for use as a structural layer in RF-MEMS devices. In this paper, we present a technology to realize suspended platinum structures by means of surface micromachining. An improved lift-off process allows patterning 1 µm Pt films on a polyimide sacrificial layer. A comparison of the characteristics and armature resonance frequencies between RF-MEMS switches with Pt armatures and AlCu0.5% alloy armatures reveals an increased thermal stability for the former up to at least 250 °C. This enables zero-level packaging of switches at relative high temperatures without affecting their performances. The lower conductivity of Pt compared to AlCu0.5% does not lead to a significant increase in RF losses. Implementing AlN as a dielectric material, the Pt-based capacitive shunt switches reported in this paper showed lifetimes in excess of 5×107 cycles under standard testing conditions.

Influence of the substrate on the lifetime of capacitive RF MEMS switches

We show for the first time that the substrate can influence the lifetime of capacitive RF MEMS switches. We demonstrate that the influence of the substrate should not be ignored. The influence of the environment on the lifetime of a switch is different when it is fabricated on two different substrates. We also present that a switch actuated with a DC voltage lower than the pull-in voltage can pull-in after some time. The goal of the performed experiment was to emphasize the charging of the substrate. The presented results help to understand the substrate charging problem.

New insights into charging in capacitive RF MEMS switches

This paper discusses dielectric charging in electrostatic RF-MEMS switches. We show that more than one charging mechanism can be present and impacts their lifetime. These different mechanisms can cancel, mitigate or enhance each otherpsilas influence on the lifetime, depending on the materials used and on the test conditions. Contrarily to the common understanding of the dielectric charging, we show that charge trapping in the dielectric interposer is not always the dominant charging mechanism leading to the failure. We finally show that bipolar actuation is not a general remedy for charging in electrostatic RF MEMS switches.

Effect of Gas Pressure on the Lifetime of Capacitive RF MEMS Switches

For the first time it is shown that the lifetime of capacitive RF MEMS switches depends on the ambient gas pressure. A change of the pressure causes a change of the electric strength of the gas and as a result electric discharging during the operation can occur. This indicates that insulator charging, the main failure mode in these switches, probably not only occurs upon contact between the top electrode and the insulator, but also without contact, due to electron emission or electrode-gap breakdown.

RF-power: driver for electrostatic RF-MEMS devices

The RF-power handling capability is an important characteristic for RF-MEMS switching devices. Apart from excessive heat dissipation, the power handling capability is mainly limited by the so-called self-biasing and/or RF-latching. These two phenomena result from the fact that the available RF-power from the source induces a non-zero electrostatic pulling force on the suspended structure. So far, self-biasing of RF-MEMS switches has always been studied assuming a perfect match of the device to the network in the ON-state (i.e. no reflection) and thus a fixed dc-equivalent rms voltage on the capacitor. If the RF-power exceeds a critical value, pull-in or self-biasing occurs. In practice, however, the assumption of the perfect match is not correct as the switch capacitance increases with increasing RF-power. This will cause a change in the reflected signal and thus a decrease in the dc-equivalent voltage source. This paper gives a new insight into the RF-power handling of RF-MEMS shunt switches and, per extension, of RF-MEMS shunt tunable capacitors. We analytically show how the negative feedback on the electrostatic force introduced by the capacitive mismatch changes the pull-in characteristics of the structure and can even stabilize it, totally avoiding the pull-in phenomenon.