Matroni Koutsoureli

Assessment of dielectric charging in electrostatically driven MEMS devices: A comparison of available characterization techniques

Abstract The present work investigates the results of different characterization methods for the dielectric charging phenomenon applicable to metal–insulator–metal (MIM) capacitors and electrostatically actuated micro-electro-mechanical-systems (MEMS). The discharge current transients (DCT), thermally stimulated depolarization current (TSDC) and Kelvin probe force microscopy (KPFM) assessment methods have been applied to either MIM capacitors or electrostatic capacitive MEMS switches or both. For the first time, the KPFM methodology has been used to create a link between the results obtained from the DCT and TSDC techniques applicable for MIM and the results from MEMS switches. The comparison shows that the application of KPFM method to MIM and MEMS leads to the same results on the electrical properties of the dielectric material. This provides a novel powerful tool for the assessment of dielectric charging for MEMS switches using MIM capacitors which have much simpler layer structure. On the other hand the TSDC method reveals a continuous distribution of relaxation time constants, which supports the dependence of relaxation time constant calculated for MEMS on the duration of the observation time window.

The Effect of Silicon Nitride Stoichiometry on Charging Mechanisms in RF-MEMS Capacitive Switches

This paper discusses the mechanisms responsible for charging of plasma enhanced chemical vapor deposition (PECVD) silicon nitride films used in the fabrication of RF microelectromechanical (MEMS) switches. Nitride films deposited at different temperatures are characterized in order to better understand the effect of deposition conditions on material stoichiometry and stress. Both RF MEMS switches and metal-semiconductor-metal capacitors with PECVD silicon nitride as the dielectric layer were fabricated and their charging mechanisms were examined. Measurements indicate that charging arises from the formation of a defect band where charge transport occurs through a Poole-Frenkel-like effect. The calculated activation energy exhibits direct relation to material stoichiometry, and therefore to the nitride bandgap. Finally, it is viewed that lower temperature nitride is less prone to dielectric charging.

A study of field emission process in electrostatically actuated MEMS switches

Abstract A study of field emission process in MEMS-based capacitor/switch-like geometries is presented. High resolution current–voltage characteristics up to breakdown have been obtained across micro-gaps in fixed–fixed Metal–Air–Metal and Metal–Air–Insulator–Metal structures. In metallic devices the I–V dependence reveals Fowler–Nordheim theory effects. In the presence of insulator the process is found to be limited by the film conductivity following Poole–Frenkel dependence. The data analysis reveals the major importance of surface asperities on the onset of the field emission process while it is also presented that charge transfer may occur between metal and insulator surfaces even in the presence of micrometer scale gaps.

Dielectric charging in capacitive microelectromechanical system switches with silicon nitride

The paper attempts to elaborate on the basic problem of dielectric charging in insulating films for microelectromechanical capacitive switches, the dependence of the film electrical properties on material stoichiometry and the uncertainty of whether the implementation of a leaky dielectric would reduce the charging effects. Silicon nitride films with stoichiometries (N/Si) ranging from 0.36 to 0.85 were assessed in metal-insulator-metal capacitors using the thermally stimulated depolarization current method and by obtaining the current-voltage characteristics. Capacitive switches were also fabricated with the same dielectric films. Results from both devices revealed an enhanced charging in the case of silicon-rich films.

Determination of bulk discharge current in the dielectric film of MEMS capacitive switches

Abstract The present work presents a new method to calculate the discharge current in the bulk of dielectric films of MEMS capacitive switches. This method takes into account the real MEMS switch with non uniform trapped charge and air gap distributions. The assessment of switches with silicon nitride dielectric film shows that the discharge current transient seems to obey the stretched exponential law. The decay characteristics depend on the polarization field’s polarity, a fact that comes along with experimental results obtained from the thermally stimulated depolarization currents (TSDC) method used in MIM capacitors.

Charge collection mechanism in MEMS capacitive switches

The present paper investigates the effect of stressing bias magnitude and stressing time on the discharging process in MEMS capacitive switches. The calculation of discharge current through the dielectric film is based on monitoring the rate of shift of bias for up-state minimum capacitance. The data analysis shows that the discharge current lies in the range of femto-Amperes and the calculated discharge time constant depends directly on the time window of observation and on the stressing conditions. Moreover the analysis reveals an increase of trapped charge that remains in the bulk of the dielectric film for very long time as the stressing bias increases. The dominant discharge process, taking place under an intrinsic field of about 103 V/cm, is found to be the hopping effect.

Properties of contactless and contacted charging in MEMS capacitive switches

Abstract The dielectric charging in MEMS capacitive switches is a complex effect. The high electric field during pull-down causes intrinsic free charge migration and dipole orientation as well as charge injection. The macroscopic dipole moment of the first two mechanisms is opposite to the one arising from charge injection. This causes partial compensation hence mitigates the overall charging and increases the device lifetime. The charging due to intrinsic free charge migration and dipole orientation can be monitored under contactless electric field application in the pull-up state. The paper investigates the characteristics of contactless charging and compares them with the ones of contacted charging. The characteristics of the discharging process that follows each charging procedure are also presented.

The effect of temperature on dielectric charging of capacitive MEMS

The present paper investigates the effect of temperature on the charging process in dielectric films of MEMS capacitive switches. The investigation includes the assessment of MIM capacitors and MEMS capacitive switches. The data analysis shows that the dielectric charging is thermally activated and the process can be described by a system with a wide distribution of relaxation times that exhibits power-law relaxation. The activation energies obtained from MIM and MEMS are attributed to different charge collection mechanisms.

Alpha particle radiation effects in RF MEMS capacitive switches

The paper investigates the effect of 5 MeV alpha particle irradiation in RF MEMS capacitive switches with silicon nitride dielectric film. The investigation included MIM capacitors in order to obtain a better insight on the irradiation introduced defects in the dielectric film. The assessment employed the thermally stimulated depolarization currents method for MIM capacitors and the capacitance–voltage characteristic for MEMS switches. Asymmetric charging was monitored in MIM capacitors due different contact electrodes and injected charge interactions.

An in depth analysis of pull-up capacitance-voltage characteristic for dielectric charging assessment of MEMS capacitive switches

Abstract The present paper aims to provide a better approach on the analysis of pull-up capacitance-voltage characteristic of MEMS capacitive switches by introducing an analytical model that takes into account the case of a real device, where the charge is not uniformly distributed at the surface of the dielectric film and the capacitor armatures are not parallel. The proposed model allows the use of capacitance-voltage characteristics derivative, which slope is directly related to the device mechanical characteristics and the stress induced during charging. The application of the model on a MEMS switch with asymmetric capacitance-voltage characteristic and on a switch with a parabolic up-state capacitance-voltage characteristic during charging and discharging processes allows the draw of some initial conclusions on the charging and the mechanical performance of the devices. Further investigation is in progress in order to extract important information on the device degradation.