L. Boudou

Voltage and temperature effect on dielectric charging for RF-MEMS capacitive switches reliability investigation

In this paper, we study the effect of stress voltage and temperature on the dielectric charging and discharging processes of silicon nitride thin films used in RF-MEMS capacitive switches. The investigation has been performed on PECVD-SiNx dielectric materials deposited under different deposition conditions. The leakage current was found to obey the Poole–Frenkel law. The charging current decay was found to be affected by the presence of defects which are generated by electron injection at high electric fields. At high temperatures power law decay was monitored. Finally, the temperature dependence of leakage current revealed the presence of thermally activated mechanisms with similar activation energies in all materials.

Charging-effects in RF capacitive switches influence of insulating layers composition

Abstract The dielectric charging is one of the major failures reducing the reliability of capacitive switches with electrostatic actuation. Then the control of the charging/discharging processes is a key factor to allow a fast recovering of the dielectric after charging. From transient current measurements on MIM capacitors it is possible to select the best material for RF-MEMS. We have studied different PECVD silicon nitride obtained under low (380 KHz), high (13.56 MHz) or mixed (380kHz/13.56MHz) frequency power supply. The conduction mechanism into the dielectrics has been deduced from current measurements on MIM capacitors. Then the film properties have been studied by infrared measurement in order to identify the chemical bond into the dielectric which can explain the charging behaviour. It was observed that low hydrogen content in the films is in good correlation with electrical quality and kinetic of the charging/discharging processes.

Kelvin probe microscopy for reliability investigation of RF-MEMS capacitive switches

In this work, we investigate the charging and reliability of interlayer dielectric materials that are used in the fabrication process of advanced RF-MEMS switches. In particular, the charge stored on the surface of a dielectric and the dynamic of this charge at nanometric scale are studied. More attention is given to the decay of the deposited charge by a variety of means: (1) surface conduction, (2) surface charge spreading due to self repulsion and (3) charge injection in the bulk of dielectric material. Kelvin force microscopy (KFM) measurements were performed for various injection time and bias voltage. These results suggest a dynamic charge and allow to predict the amount of charge injected into the dielectric.

Silver nanoparticles as a key feature of a plasma polymer composite layer in mitigation of charge injection into polyethylene under dc stress

The aim of this work is to limit charge injection from a semi-conducting electrode into low density polyethylene (LDPE) under dc field by tailoring the polymer surface using a silver nanoparticles-containing layer. The layer is composed of a plane of silver nanoparticles embedded in a semi-insulating organosilicon matrix deposited on the polyethylene surface by a plasma process. Size, density and surface coverage of the nanoparticles are controlled through the plasma process. Space charge distribution in 300 μm thick LDPE samples is measured by the pulsed-electroacoustic technique following a short term (step-wise voltage increase up to 50 kV mm−1, 20 min in duration each, followed by a polarity inversion) and a longer term (up to 12 h under 40 kV mm−1) protocols for voltage application. A comparative study of space charge distribution between a reference polyethylene sample and the tailored samples is presented. It is shown that the barrier effect depends on the size distribution and the surface area covered by the nanoparticles: 15 nm (average size) silver nanoparticles with a high surface density but still not percolating form an efficient barrier layer that suppress charge injection. It is worthy to note that charge injection is detected for samples tailored with (i) percolating nanoparticles embedded in organosilicon layer; (ii) with organosilicon layer only, without nanoparticles and (iii) with smaller size silver particles (<10 nm) embedded in organosilicon layer. The amount of injected charges in the tailored samples increases gradually in the samples ranking given above. The mechanism of charge injection mitigation is discussed on the basis of complementary experiments carried out on the nanocomposite layer such as surface potential measurements. The ability of silver clusters to stabilize electrical charges close to the electrode thereby counterbalancing the applied field appears to be a key factor in explaining the charge injection mitigation effect.

Kelvin force microscopy characterization of charging effect in thin a-SiOxNy:H layers deposited in pulsed plasma enhanced chemical vapor deposition process by tuning the Silicon-environment

Results from a study on the charging effect of a-SiOxNy:H thin layers are presented in this paper. Issues related to structural and electrical characterization of these layers are discussed. Spectroscopic ellipsometry was used to determine accurately the layer thickness and their optical properties, while the Kelvin Force Microscopy (KFM) was applied to characterize the local electrical properties of the layers. Obtained results reveal that by tuning the Si-environment in a-SiOxNy:H thin dielectric layers, deposited in plasma assisted process, a strong modification of the surface and volume charge conduction can be achieved. Particularly, increasing Si-content in the a-SiOxNy:H layers rises the volume conduction and charges retention. Thus, local electrical properties of thin dielectric layers can be engineered in order to meet specific requirements.

Dielectric layers for RF-MEMS switches: Design and study of appropriate structures preventing electrostatic charging

Both multi-layer structures with discrete levels and with continuous ones represent structural and dielectric properties that are adapted to prevent the electrostatic charging in the dielectric layer of RF-MEMS capacitive switches. The role of interfaces in the multi-layer with discrete levels for charge evacuation will be the next step in our study.

Charge build-up and transport in electron beam irradiated polymers in a new irradiation chamber

In order to study the behavior of dielectric materials under an electronic bombardment, a new irradiation chamber has been designed and lately constructed in the laboratory. In a near future, this chamber will be equipped with various experimental set-up allowing measurements in-situ. The aim is to combine complementary information to get a better knowledge on charge transport, storage and release in insulating materials in relation to the electron beam energy and flux. In this paper, we briefly describe the irradiation chamber, and we present our first results obtained by Pulse Electro-Acoustic (PEA) method and by Focussed Laser Intensity Modulated Method (FLIMM) on Polytetrafluoroethylene (PTFE) films after irradiation under a low energetic electron beam.

Towards 3D charge localization by a method derived from atomic force microscopy: the electrostatic force distance curve

Charges injection and accumulation in the dielectric remains a critical issue, mainly because these phenomena are involved in a great number of failure mechanisms in cables or electronic components. Achieving a better understanding of the mechanisms leading to charge injection, transport and trapping under electrical stress and of the relevant interface phenomena is a high priority. The classical methods used for space charge density profile measurements have a limited spatial resolution, which prevents them being used for investigating thin dielectric layers or interface processes. Thus, techniques derived from atomic force microscopy (AFM) have been investigated more and more for this kind of application, but so far they have been limited by their lack of in-depth sensitivity. In this paper a new method for space charge probing is described, the electrostatic force distance curve (EFDC), which is based on electrostatic force measurements using AFM. A comparison with the results obtained using kelvin force microscopy (KFM) allowed us to highlight the fact that EFDC is sensitive to charges localized in the third-dimension.

Challenges in probing space charge at sub-micrometer scale

An overview of current limitations and challenges with techniques, based either on acoustic or thermal perturbation, providing charge density profiles within insulations, is presented. Even though the resolution could be somewhat improved, technical limitations readily appear, related to the bandwidth of signals to be detected and to the sensitivity. Instead, our purpose here is to exploit near field techniques derived from AFM - Atomic Force Microscopy-. A booming of the availability and versatility of equipments is observed today. A spatial resolution of some tens of nanometers is accessible for charge detection which therefore lets the possibility to investigate selectively regions with specific properties. The measuring conditions and operating mode for both the sensitivity and spatial resolution of the techniques are addressed and examples of application of these techniques to charge detection in insulating materials are presented.

Numerical simulation of thermo-stimulated depolarization currents in polyethylene films

Complementary thermo-stimulated depolarization current (TSDC) and PEA measurements have been performed on a low density polyethylene. A bipolar transport model developed formerly to reproduce the behavior of space charge in a low density polyethylene under electrical stress has been adapted to simulate TSDC spectra. The model is one dimensional (sample thickness). Temperature effects have been introduced by using a hopping type of mobility, detrapping and charge injection also having temperature dependencies. The model shows that the surface of the TSDC spectra largely underestimates the total stored charge within the sample. We show that the model is able to reproduce qualitatively some features of the TSDC spectra which appears promising for the future.