Dielectric charging is one of the main problems leading to failure of capacitive RF MEMS switches. In this work
phosphorus or boron ions were implanted into dielectric layer by ion implantation. After dielectric layer modification by
ion implantation, we focus on investigation of the mechanisms of the charge accumulation and recombination after the
sample electrically stressed with 80 V for 30 seconds. A
Metal-Insulator-Semiconductor (MIS) capacitor structure is
used for such an investigation. Silicon nitride films as the insulator in MIS structure were deposited by LPCVD process.
The space charge accumulation in the silicon nitride film can be characterized by Capacitance-Voltage (C-V)
measurement. Because of the ionization of the gas in the operating environment of the switch, ion injection by actuation
voltage during the operation of the RF MEMS switch will play the role to enhance the charge accumulation in the
dielectric layer. Our work offers a principle to understand the effect of the operating environment to the lifetime and
reliability of the RF capacitive MEMS switches.
RF MEMS capacitive switches hold great promise in commercial, aerospace, and military applications. However,
their commercialization is hindered by reliability concerns: charging effect in the dielectric layer can cause irreversible
stiction of the actuating part of the switch. Presently, a popular method to investigate the charging/discharging in the
dielectric layer is to measure an actual RF MEMS capacitive switch, which means a high experimental cost in fabricating
MEMS switch devices.
In this paper, a Metal-Insulator-Semiconductor (MIS) capacitor is used to investigate the charge accumulation in
the dielectric layer of RF MEMS switches. By measuring the capacitance versus voltage (C-V) curves of MIS capacitor
after voltage stressing, the dielectric charging/discharging characteristics are obtained. The experiment results indicate
that the injected charges from the metal bridge in RF MEMS switches are responsible for stiction phenomena. In SiNx
dielectric, the hole capture is more favored over electron capture, and the trapped charges tend to inhibit the charges
further injecting. The effects of the actuation voltage waveform on the charge accumulation in the dielectric layer were
investigated. It is verified that the tailored actuation voltage waveforms can be used to improve the reliability of RF
MEMS capacitive switches.
In this paper, our work is focusing on investigating the mechanisms of the charge accumulation in dielectric layer of RF
MEMS capacitive switches. In our experiments, silicon-nitride and silicon-oxide composite films, e.g., SiO<sub>2</sub>+Si<sub>3</sub>N<sub>4</sub> and
SiO<sub>2</sub>+Si<sub>3</sub>N<sub>4</sub>+SiO<sub>2</sub> films are chosen as the dielectric layers for study. The composite films were prepared by thermal
oxidation and PECVD process. The Metal-Insulator-Semiconductor (MIS) structure was produced by using the
composite films as the dielectric layer. The capacitance versus voltage (C-V) measurement is employed to study the
space charge injection and relaxation process in the composite films. The results show that the charge accumulation can
be reduced by using the composite films structure.
For higher-power-handling RF MEMS switches, the design of the switch is based on fixed-fixed beam capacitive structure with electrostatic actuation. Such RF MEMS switches are perceived to be unreliable because of the stiction and screening of the beam caused by charge accumulation in the dielectric layer. The research effort for a robust RF MEMS solution has been made for more than a decade. In this paper the models for stiction and screening caused by charge accumulation have been reviewed. As the first part of this paper, the possible charging mechanisms will be described, such as, 1) the dielectric charging arises from charges distributed throughout the dielectric material, 2) the presence of charges at the dielectric interface. In order to avoid the charge accumulation, trapped charges in the dielectric layer have to quickly vanish. Relaxing mechanisms of short time must be created inside of the dielectric for quick charge recombination. The second part of this paper will report the recent effort to create relaxing mechanisms of short time by using, such as doping dielectric, nano-composite dielectrics, or multi-layer stack of dielectric. Actuation wave form dependence of the charge accumulation will be also presented.