Usama Zaghloul

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

Abstract 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.

On the influence of environment gases, relative humidity and gas purification on dielectric charging/discharging processes in electrostatically driven MEMS/NEMS devices

In this paper, we investigate the impact of environment gases and relative humidity on dielectric charging phenomenon in electrostatically actuated micro- and nano-electromechanical systems (MEMS and NEMS). The research is based on surface potential measurements using Kelvin probe force microscopy (KPFM). Plasma-enhanced chemical vapor deposition (PECVD) silicon nitride films were investigated in view of applications in electrostatic capacitive RF MEMS switches. Charges were injected through the atomic force microscope (AFM) tip, and the induced surface potential was measured using KPFM. Experiments have been performed in air and in nitrogen environments, both under different relative humidity levels ranging from 0.02% to 40%. The impact of oxygen gas and hydrocarbon contaminants has been studied for the first time by using different gas purifiers in both air and nitrogen lines. Voltage pulses with different bias amplitudes have been applied during the charge injection step under all investigated environmental conditions in order to investigate the effect of bias amplitude. The investigation reveals a deeper understanding of charging and discharging processes and could further lead to improved operating environment conditions in order to minimize the dielectric charging. Finally, the nanoscale KPFM results obtained in this study show a good correlation with the device level measurements for capacitive MEMS switches reported in the literature.

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.

A systematic reliability investigation of the dielectric charging process in electrostatically actuated MEMS based on Kelvin probe force microscopy

This paper presents a comprehensive investigation for the dielectric charging problem in electrostatically actuated microelectromechanical system (MEMS) devices. The approach is based on Kelvin probe force microscopy (KPFM) and targets, in this specific paper, thin PECVD silicon nitride films for electrostatic capacitive RF MEMS switches. KPFM has been employed in order to mimic the potential induced at the dielectric surface due to charge injection through asperities. The effect of dielectric thickness has been investigated through depositing SiNx films with different thicknesses. Then, in order to simulate the different scenarios of dielectric charging in real MEMS switches, SiNx films have been deposited over thermally grown oxide, evaporated gold and electroplated gold layers. Also, the effect of the deposition conditions has been investigated through depositing dielectric films using low and high frequency PECVD methods. The investigation reveals that thin dielectric films have larger relaxation times compared to thick ones when the same injection bias is applied, independently of the substrate nature. For the same SiNx film thickness, the decay time constant is found to be smaller in dielectric films deposited over metallic layers compared to the ones deposited over silicon substrates. Finally, the material stoichiometry is found to affect the surface potential distribution as well as the relaxation time constant.

Nanoscale characterization of different stiction mechanisms in electrostatically driven MEMS devices based on adhesion and friction measurements

In this work, for the first time different stiction mechanisms in electrostatic micro-electromechanical systems (MEMS) switches were studied. In these devices stiction can be caused by two main mechanisms: dielectric charging and meniscus formation resulting from the adsorbed water film between the switch bridge and the dielectric layer. The effect of each mechanism and their interaction were investigated by measuring the adhesive and friction forces under different electrical stress conditions and relative humidity levels. An atomic force microscope (AFM) was used to perform force-distance and friction measurements on the nanoscale. A novel technique was proposed to measure the induced surface potential over the dielectric surface and was used to explain the obtained adhesive and friction results. The evolution of adhesive force with time was monitored in order to study the charging/discharging processes in the dielectric film. The assessment methodology is employed for application in RF-MEMS switches and could be extended to other electrostatic MEMS devices. The study provides an in-depth understanding of different stiction mechanisms, and explanation for the literature reported device level measurements for electrostatic capacitive MEMS switches.

Nanoscale characterization of the dielectric charging phenomenon in PECVD silicon nitride thin films with various interfacial structures based on Kelvin probe force microscopy

This work presents a novel characterization methodology for the dielectric charging phenomenon in electrostatically driven MEMS devices using Kelvin probe force microscopy (KPFM). It has been used to study plasma-enhanced chemical vapor deposition (PECVD) silicon nitride thin films in view of application in electrostatic capacitive RF MEMS switches. The proposed technique takes the advantage of the atomic force microscope (AFM) tip to simulate charge injection through asperities, and then the induced surface potential is measured. The impact of bias amplitude, bias polarity, and bias duration employed during charge injection has been explored. The influence of various parameters on the charging/discharging processes has been investigated: dielectric film thickness, SiN(x) material deposition conditions, and under layers. Fourier transform infrared spectroscopy (FT-IR) and x-ray photoelectron spectroscopy (XPS) material characterization techniques have been used to determine the chemical bonds and compositions, respectively, of the SiN(x) films being investigated. The required samples for this technique consist only of thin dielectric films deposited over planar substrates, and no photolithography steps are required. Therefore, the proposed methodology provides a low cost and quite fast solution compared to other available characterization techniques of actual MEMS switches. Finally, the comparison between the KPFM results and the discharge current transients (DCT) measurements shows a quite good agreement.

Kelvin probe force microscopy-based characterization techniques applied for electrostatic MEMS/NEMS devices and bare dielectric films to investigate the dielectric and substrate charging phenomena

In this study, two different characterization techniques based on Kelvin probe force microscopy (KPFM) have been used to investigate the dielectric and substrate charging in electrostatic micro- and nano-electromechanical systems (MEMS and NEMS). The first technique (KPFM-MEMS) has been employed to study the discharging process on a microscopic scale in a charged MEMS dielectric film. This has been performed by monitoring the surface potential decay with time of charged PECVD silicon nitride films implemented in electrostatic capacitive MEMS switches. The second methodology, KPFM-thin films (KPFM-TF), has been applied to investigate the charging/discharging processes in bare SiNx films as well as the substrate charging phenomenon. It makes use of the atomic force microscope tip to simulate charge injection through a single asperity, and then measure the induced surface potential. The influence of the SiNx film thickness and deposition conditions has been studied. Moreover, the impact of bias amplitude and...

Nanotribology-based novel characterization techniques for the dielectric charging failure mechanism in electrostatically actuated NEMS/MEMS devices using force-distance curve measurements.

The work presents a comprehensive package of novel nanoscale characterization techniques to study dielectric charging in electrostatic nano- and microelectromechanical systems (NEMS and MEMS). The proposed assessment methodologies are based on the force-distance curve (FDC) measurements performed using an atomic force microscope (AFM) to measure, for the first time, the induced surface potential and adhesive force over charged dielectric films. They were employed to study plasma enhanced chemical vapor deposition (PECVD) silicon nitride films for application in electrostatic capacitive RF MEMS switches. Three different techniques were introduced including the application of FDC measurements to study charging in bare SiN(x) films, metal-insulator-metal (MIM) capacitors, and MEMS switches. The results from the three methods were correlated and compared with the published data from other characterization techniques, mainly charge/discharge current transient (C/DCT) and Kelvin probe force microscopy (KPFM). The unique advantages of the proposed FDC-based characterization techniques are twofold. First, they can measure the multiphysics coupling between the dielectric charging phenomenon and tribological issues at the interface between the switch bridge and the dielectric surface. Second, the FDC-based techniques can measure larger levels of induced surface potential over charged dielectric films which results from the high electric field normally used to actuate MEMS switches. Based on the proposed FDC techniques, the influence of several parameters on dielectric charging/discharging processes was investigated: the dielectric film thickness, deposition conditions, substrate, and electrical stress conditions.

Effect of Deposition Gas Ratio, RF Power, and Substrate Temperature on the Charging/Discharging Processes in PECVD Silicon Nitride Films for Electrostatic NEMS/MEMS Reliability Using Atomic Force Microscopy

The dependence of the electrical properties of silicon nitride, which is a commonly used dielectric in nano- and micro-electromechanical systems (NEMS and MEMS), on the deposition conditions used to prepare it and, consequently, on material stoichiometry has not been fully understood. In this paper, the influence of plasma-enhanced chemical vapor deposition conditions on the dielectric charging of films is investigated. The work targets mainly the dielectric-charging phenomenon which constitutes a major failure mechanism in electrostatically driven NEMS/MEMS devices and particularly in capacitive MEMS switches. The charging/discharging processes are studied using two nanoscale characterization techniques: Kelvin probe force microscopy (KPFM) and, for the first time, force-distance curve (FDC) measurements. KPFM is used to investigate dielectric charging at the level of a single asperity, while FDC is employed to measure the multiphysics coupling between the charging phenomenon and tribological issues, mainly meniscus force. The electrical properties of the films obtained from both techniques show a very good correlation. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy material characterization techniques are also used to determine the compositions and chemical bonds, respectively, of the films. An attempt to correlate between the chemical and electrical properties of films is made.

Different stiction mechanisms in electrostatic MEMS devices: Nanoscale characterization based on adhesion and friction measurements

In this work, different stiction mechanisms in electrostatic micro-electromechanical systems (MEMS) and particularly in RF-MEMS switches were studied for the first time. In these devices stiction can be caused by two main mechanisms: dielectric charging and meniscus formation resulting from the adsorbed water film between the switch bridge and the dielectric layer. The effect of each mechanism and their interaction were investigated by measuring the adhesive and friction forces under different electrical stress conditions and relative humidity levels. An atomic force microscope (AFM) was used to perform force-distance and friction measurements on the nanoscale. The study provides an in-depth understanding of different stiction mechanisms, and explanation for the literature reported lifetime measurements for electrostatic capacitive MEMS switches.