On the reliability of electrostatic NEMS/MEMS devices: Review of present knowledge on the dielectric charging and stiction failure mechanisms and novel characterization methodologies

This paper reviews the state of the art knowledge related to critical failure mechanisms in electrostatic micro- and nano-electromechanical systems (MEMS and NEMS) which are the dielectric charging and stiction. It describes also the recent employed nanoscale characterization techniques for these phenomena based on Kelvin probe force microscopy (KPFM) and force–distance curve measurements. The influence of relative humidity and dielectric deposition conditions on the charging/discharging processes is discussed. Moreover, different stiction mechanisms induced by electrostatic force and/or meniscus formation are analyzed. Finally, novel characterization methods are presented and used to correlate between the results from MEMS devices and metal–insulator–metal (MIM) capacitors. These methods are employed in view of application in electrostatic capacitive MEMS switches and could be easily extended to explore other NEMS/MEMS devices. The study provides an accurate understanding of the charging and stiction related failure mechanisms, presents guidelines for a proper packaging environment, and reveals precise explanations for the literature reported device level measurements of electrostatic MEMS devices.     

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.     

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

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.     

Effect of deposition conditions on charging processes in SiNx: Application to RF-MEMS capacitive switches

The paper presents a systematic investigation of dielectric charging in low temperature silicon nitride for RF-MEMS capacitive switches. The dielectric charging is investigated with the aid of Metal-Insulator-Metal (MIM) capacitors with different thickness dielectric film and symmetric and asymmetric metal contacts. The experimental results demonstrate that the charging process is almost symmetric in low temperature deposited silicon nitride. Experiments performed in both MIM and MEMS reveal that the charging process is strongly affected by temperature. Specifically at high temperatures the charging rate increases exponentially with temperature.     

Modeling of dielectric charging in electrostatic MEMS switches

The most important failure mechanism for electrostatic MEMS switches is dielectric charging, which contributes to a significant reduction of the device lifetime. In this study the correlation between the dielectric properties and the switch lifetime is evaluated. The conduction mechanism and trapping kinetics for two types of PECVD SiN_x are determined by I–V sweeps and constant-current injections from Metal–Insulator–Metal (MIM) capacitors. This type of procedure is used as a basis for modeling the charge build-up in a switch. Despite significant differences between the dielectrics, in terms of leakage current and trapping properties, the numerical model of charge build-up fits well with experimental data. We conclude that the switch lifetime can be correlated with the trapping properties of the dielectric itself.     

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

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.     

Dielectric Relaxation of MIM Capacitor and Its Effect on Sigma-Delta A/D Converters

Dielectric relaxation of capacitors is one of the error sources when determining the accuracy of analog sampled-data systems that are based on charge storage. To perform an accurate characterization of the dielectric relaxation of metal–insulator–metal (MIM) capacitor, techniques based on the voltage recovery principle and the Curie Von Schweidler discharge current approach are developed. To model the dielectric relaxation of the MIM capacitor, Dow's model is selected. An algorithm for the model parameter extraction on the Curie Von Schweidler current has been developed, which shows the phenomenon of dielectric relaxation in detail and is very fast to determine the parameters. Based on the measurement data, a set of model parameters is extracted and verified, which approximates the Curie Von Schweidler law over a sufficiently wide interval of time constants. To study the effect of the dielectric relaxation on the circuit performance of high resolution sigma-delta ADC, a 12-b incremental ADC has been selected as an example for simulation. The simulation results show that the effect of the dielectric relaxation on the performance of the 12-b ADC is not significant. We show that the major reason for this is that the noise shaping which is enforced by the integrators in $SigmaDelta$ -ADC is almost not affected by the dielectric relaxation phenomenon.      

Structure dependent charging process in RF MEMS capacitive switches

The paper investigates the dependence of charging process on the dielectric charging of radiation induced defects in Si₃N₄ and SiO₂ dielectric films, which are used in RF-MEMS switches. The radiation has been performed with 5 MeV alpha particles. The assessment has been carried out in Metal-Insulator-Metal capacitors with the thermally stimulated depolarization currents and discharge current transient methods. This allowed monitoring the defects introduction as a function of radiation fluence. The defects electrical characteristics that are the activation energy and corresponding depolarization time constant were determined from the evolution of the thermally stimulated current spectra and the transient response of discharge currents at different temperatures.     

Measurement and analysis of substrate leakage current of RF mems capacitive switches

This paper focused on a new direction of study on leakage current called substrate charge injection. The substrate leakage current of capacitive RF micro-electro-mechanical-system (MEMS) switches was measured, and the conduction mechanism was estimated. The study of the leakage current conduction mechanisms of the substrate dielectric film shows that leakage is mainly induced by hopping conduction at low electric fields, whereas both Schottky emission and hopping conduction may contribute to the leakage current at high fields. The quantitative relationship between the substrate leakage current and the dielectric layer leakage current was also determined for the first time. In the case of low drive voltage (0–30 V), the substrate leakage current significantly contributes to the total leakage current. Results show that the charging properties of the substrate should not be neglected at low drive voltage because such properties could significantly affect the functionality and reliability of RF MEMS switches.     

Charging of radiation induced defects in RF MEMS dielectric films

The paper investigates the charging of radiation induced defects in Si₃N₄ and SiO₂ dielectric films, which are used in RF-MEMS switches. The radiation has been performed with 5MeV alpha particles. The assessment has been carried out in Metal-Insulator-Metal capacitors with the thermally stimulated depolarization currents and discharge current transient methods. This allowed monitoring the defects introduction as a function of radiation fluence. The defects electrical characteristics that are the activation energy and corresponding depolarization time constant were determined from the evolution of the thermally stimulated current spectra and the transient response of discharge currents at different temperatures.     

静电式RF MEMS开关的可靠性

MEMS开关是最常见的RF MEMS控制元件,是RF结构中一个关键的MEMS器件。长期可靠性是目前制约MEMS开关商业化进程中的一个主要问题。主要综述了静电式RF MEMS开关可靠性的新进展。欧姆式开关通常由于黏附或接触电阻的增大而失效,电容式开关的主要失效机理则与电介质层的充电有关。接触材料的选择是决定欧姆开关可靠性最重要的一个因素,"主动断开/被动接触"MEMS开关适用于软金属材料欧姆接触的可靠性要求。改善电容式开关可靠性的途径是改善介电层、优化驱动电压波形等以减小介质层的充电。     

Dielectric charging induced drift in micro device reliability-a review

The movement or migration of charges in dielectric materials like silicon oxide, silicon nitride and glass, is recognized as one of the most significant causes of drift instability of MEMS devices which utilize electrostatic capacitive methods for sensing and driving. This paper reviews the current researches on the characteristics of drift phenomenon of three micro capacitive devices, micro switches, micro resonators and micro mirrors. The dielectric charging forms including polarization, ion injection and charge migration are presented in detail to explain the process and mechanism of how the charging effects gives rise to the drift of performance and influence the reliability of micro systems, and then the corresponding solutions to overcome specific drift issues are proposed based on the essential conditions needed to cause dielectric charging.     

Device physics of capacitive MEMS

Using interferometric modulator (IMOD) MEMS-based technology as a typical example, we give an overview of key device concepts of capacitive micro-electro-mechanical systems (MEMS). We discuss basic electromechanical physics of the device, both in the ideal case of no charging in the dielectric and the more realistic scenario with charging. The dielectric stack is a critical element of capacitive MEMS. A significant part of the paper is dedicated to the role of the dielectric and electronic transport effects during device operation. The similarities and differences between dielectrics in MEMS and gate oxides in semiconductor MOS and MIM structures are highlighted. Several experimental techniques to study charge transport in the MEMS dielectric stack are reviewed following a discussion on how the dynamics of these electronic effects can be modeled. Finally, we comment on the importance of surface and interfaces on MEMS performance and reliability as well as fundamental studies of their properties.     

Effect of size and plasma treatment and the application of Weibull distribution on the breakdown of PECVD SiNx MIM capacitors

The dielectric breakdown field is one of the important concerns for device reliability. The breakdown of dielectric is originated at a fatal flaw that grows to cause failure and can be explained by the weakest-link theory. In this study, metal-insulator-metal (MIM) capacitors with plasma enhanced chemical vapor deposited (PECVD) SiN_x are prepared. Ammonia (NH₃) plasmas are applied after the deposition of the dielectric SiN_x. The Weibull distribution function, which is based on the weakest-link theory, is employed to analyze the effect of the electrode area as well as the plasma treatment on the breakdown of the MIM capacitors. The time dependent dielectric breakdown testing indicates a decrease in both the leakage current and the lifetime of the MIM capacitors treated with plasma. Possible dielectric degradation mechanisms are explored.     

Nanostructured metal–insulator–metal capacitor with anodic titania

This paper presents fabrication and electrical characterization of barrier type TiO₂ metal–insulator–metal (MIM) capacitor using anodization. Polarization process, conduction mechanisms, and structural properties are studied in detail. We found that the anodization voltage played a major role in electrical and structural properties of the thin film. The barrier type anodic TiO₂ is suggested as a dielectric material for high-performance MIM capacitors.     

RF MEMS工艺中牺牲层的去除方法研究

研究了MEMS开关聚酰亚胺牺牲层的去除，通过添加少许碳粉，能够减少刻蚀时间。讨论了在这种刻蚀情况下刻蚀温度和刻蚀时间之间的关系。此研究应用在表面微细加工工艺中，对MEMS加工具有重要的参考价值。该研究还可应用于RF MEMS开关、可调电容、高Q值悬臂电感、MOSFET等制造工艺中。     

Impact of Humidity on Dielectric Charging in RF MEMS Capacitive Switches

A novel technique is used to distinguish the charging of the surface from that of the bulk of the dielectrics of different types of RF MEMS capacitive switches under different electric fields and humidity levels. In general, bulk charging dominates in dry air, while surface charging increases linearly with increasing humidity. Under comparable electric fields and humidity levels, switches made of silicon dioxide are less susceptible to surface charging than switches made of silicon nitride. These quantitative results not only underscore the importance of packaging the switches in a dry ambient atmosphere, but also validate the novel technique for evaluating the effectiveness of dielectric preparation and packaging.      

Time dependent breakdown characteristics of parylene dielectric in metal–insulator–metal capacitors

In this work, we present reliability results of MIM (Metal–Insulator–Metal) capacitors fabricated with parylene as the dielectric, deposited at room temperature. We have evaluated the time dependent dielectric breakdown (TDDB) of parylene-based MIM capacitors as a function of constant DC voltage stress, area and dielectric thickness of the capacitor. Mean-time-to-failure (MTTF) of parylene evaluated at different stress voltages shows a power law distribution over the applied voltage range and device area, with MTTF driven by the number of defects. Defect density in the parylene capacitors is also reported and is calculated to be ～1.2 × 10³ defects/cm².     

Dielectric charging in silicon nitride films for MEMS capacitive switches: Effect of film thickness and deposition conditions

The paper presents a systematic investigation of the dielectric charging and discharging process in silicon nitride thin films for RF-MEMS capacitive switches. The SiN films were deposited with high frequency (HF) and low frequency (LF) PECVD method and with different thicknesses. Metal–Insulator–Metal capacitors have been chosen as test structures while the Charge/Discharge Current Transient method has been used to monitor the current transients. The investigation reveals that in LF material the stored charge increases with the film thickness while in HF one it is not affected by the film thickness. The dependence of stored charge on electric field intensity was found to follow a Poole–Frenkel like law. Finally, both the relaxation time and the stored charge were found to increase with the electric field intensity.     

Prediction of dielectric reliability from I–V characteristics: Poole–Frenkel conduction mechanism leading to √E model for silicon nitride MIM capacitor

Two assumptions lead to a correlation between the leakage mechanism of a dielectric and dielectric reliability: the degradation of the dielectric is a direct cause of the leakage current flowing through the dielectric and breakdown occurs after a critical charge has been forced through the dielectric. The field and temperature dependence of the leakage current mechanism then determine the voltage acceleration factor and the activation energy of TDDB experiments. This simple physical model describes the reliability of metal insulator metal (MIM) capacitors with PECVD SiN remarkably well. The current conduction mechanism is described by Poole–Frenkel theory, leading to a √E dependence of the time to breakdown on the applied electric field. The model predicts correctly the voltage acceleration factor and its temperature dependence and the activation energy.