MEMS World - Electrostatic actuators

It can take several books to give an exhaustive description of all MEMS electrostatic actuators things to know, not speaking about electrostatics phenomenon itself. This page just intends to describe the principle and benefits of electrostatic actuation at microscopic scale, as well for direct parallel plates electrostatic actuation as comb-drive microactuators.

Electrostatic force

Any electric charge creates around it an electric field. Any electric field applies a force to any charged particle. This principle, widely known since Maxwell's era, has not been so much used during the past decades, but MEMS have a high interest in using electrostatic actuators:
The main problem of electrostatic effect is that it decreases with the square of the distance between the two charged bodies. In microscopic scale, this is a huge advantage, because most of the structures have a very low aspect ratio (i.e. width and length are large before thickness and gap in z direction), so the distance between bodies is really very small.
Electrostatics is the most widely used force in the design of MEMS. In industry, it is used in microresonators, switches, micromirrors, accelerometers, etc. Almost every kind of microactuator has one or more electrostatic actuation based version.

Pros and cons

Electrostatic force depends largely on the size of the structures and the distance between electrodes. So, for large electrode surface compared to distance to travel, electrostatic actuation has a large advantage. But the equation of the force gives a dependancy to the square of the distance. This means that the longer the distance is, the higher the actuation voltage is. This is one of the main problem with this physics principle: actuation voltage are often quite high, easily reaching tens, and even hundreds of volts to be used. High voltages are easy to get on a large device, but not on a very compact integrated system.
An interesting point is the fact that the electric circuit is capacitive, meaning the power consumption is very low, despite of any high voltage. Very few current is needed to load the capacity formed by electrodes.
Another consideration to take care of is the electric field itself: the nature of the material between electrodes: water, for example, is conductive at low frequencies, so electrostatic actuation cannot be used in these conditions. Void and neutral gases are the best environments.
Finally, the hysteresis behaviour of straight actuators can be as well an advantage and a problem, depending on the application. It reduces sensitivity of devices to electrical noise, but it also means larger voltage variation for a complete actuation cycle when pull-in/pull-out is desired.

A detail of the calculation of the electrostatic actuation can be found in the electrostatics section.

This example shows an electrostatic actuator in which one plate is electrostatically attracted toward another. You can consider this an electrostatic tunable capacitor if you wish, since some of them use the same principle. But remember real systems are often more complicated!

Electrostatic actuation systems

Straight actuation

By straight actuation, I mean actuation between two electrodes in which the mobile parts moves along the electric field paths. This is the most simple actuation technique you can imagine, though the architectures used are often not so simple to optimize it.

The tunable capacitor is an example of straight actuation. But there are others. Microswitches devices make a large use of electrostatic actuation, depending on the characteristics required for the device. Micromirrors devices are almost all based on electrostatic force.

A simple electrostatic microswitch
Example of application: Schematics of a simple electrostatic microswitch

In a straight actuation, the exact movement of the mobile parts is ruled by the electric field path and the mechanical constrains of the structure. So, tunable capacitor will translate along a straight line, micromirrors will rotate around their anchor axis, etc. The actuation is not always really straight, but it is as well as possible. This technique is probably the most efficient in terms of forces, and so required voltage.

The straight actuation is the one in which pull-in/pull-out hysteresis cycle can be met.

Comb-drive actuators

Another widely used architecture for electrostatic actuation is a lateral translation of the mobile parts. This is mostly achieved with comb-drive actuators

Comb-drive actuator
Principle of comb-drive actuation
The blue part is fixed and anchored to the substrate. The red part is free to move, except at the end where it is anchored to the substrate, so that the actuator can come back to its initial position thanks to the spring force.

In this configuration, the force is equally and symmetrically applied on both side of beams, and the mobile part moves along the beams direction.

The total displacement is rather shorter than what is possible in straight actuation, but the force is constant, meaning an easier control of the displacement. This kind of system offers a very high precision level, and a simpler electronic control. Systems like microaccelerometers make high use of comb-drive.

In theory, comb-drive would require higher voltage than straight actuation because of the electric-field direction being different than displacement one. But most of the time, theses structures are geometrically optimized, so the gap between electrodes and the distance to travel are also shorter.

In a comb-drive, there is no pull-in effect, unless a design error make electrodes reach each others. The displacement is linear, contrary to straight actuation. But the design is limited by the needs to place electrodes beams opposite to each other. Designer must also take care of the end stop to avoid contact between mobile part and fixed part, and to keep forces quite symmetric so that the direction of the displacement is kept as planned.

Microresonators

Microresonators are structures not planned to be displaced, but, as their name tells, to vibrate. They're planned to replace electronic resonators, being used in electronic filter systems, so select a particular frequency chosen amongst several ones. This is highly used in telecommunications. For example, in mobile phone communication: each telephone uses a precise frequency included in a range depending on the protocol. The filter must keep only the chosen frequency, so that you're not disturbed by other people conversations during your own calls!

All mechanical systems have specific resonance frequencies. Most of time, only the first one are useful. Think of it as guitar cords: if you stimulate it, it is, from a frequency point of view, as if you were applying a signal containing all frequencies at once on the cord, but only one note comes out: it is the eigen frequency of the cord.

So, microresonators use mechanical vibrating parts to filter signals so that only one frequency, the eigen frequency of the structure, is kept. The application is for electronic signal treatment, so, there must be a way to convert electrical signal into mechanical stimulation, and then back again mechanical vibration into electrical signal. This way is the electrostatic principle.

The top schematics is always the same: the vibrating part includes as well the electrode for stimulation, and the electrode for measurement. Then, a lot of architectures are under studies in research laboratories, including different shapes and electrodes configuration for mechanical and electrical parts.

Microresonators photos
Photos of several microresonators architectures from the IEMN Silicon Microsystem Group
Design and fabrication by Emmanuel Quevy
All rights reserved

The vibration is made by the incoming electrical signal. Then, the amplitude of the vibration depends on how close the input frequency is to the eigen value. On the output, a polarization (constant voltage) between the resonator and the output electrode allows the measurement of the amplitude of vibration. Since the amplitude difference obtained between the eigen frequency and other ones is very high, the electrical output signal for any non-selected frequency is considered as neglectible, while the eigen frequency signal is kept.
Note that this is a quite simple explanation of the behaviour of the system. In real development a LOT of parameters are taken in account.

The photos show microresonators architecture vibrating in the range of tens of MHz, that corresponds to Intermediate Frequencies (IF). Intermediate frequencies are a convenient way of treating a signal between low frequency (ex: human voice frequency) and really high frequencies (at which mobile phones communicate). The distance between mobile parts and fixed parts here is about 60nm.

Microresonators should enter the market quite soon from now. They will replace electronic devices, especially in mobile devices, and allow greater autonomy and more integrated functions thanks to the save of space and power consumption.

Finally, you can find an example of a real developed straight actuator in my PhD project on micro-extractor, in which the microactuator is in fact a combined thermal-electrostatic actuator.