domingo, 30 de mayo de 2010

What are Microsystems / MEMS?


MEMS is an abbreviation for Micro Electro Mechanical Systems. This is a rapidly emerging technology combining electrical, electronic, mechanical, optical, material, chemical, and fluids engineering disciplines. As the smallest commercially produced "machines", MEMS devices are similar to traditional sensors and actuators although much, much smaller. e.g. Complete systems are typically a few millimeters across, with individual features / devices of the order of 1-100 micrometers across.

Electron micrscope image of a MEMS electromechanical lock. Courtesy Sandia National Labs.
MEMS devices are manufactured either using processes based on Integrated Circuit fabrication techniques and materials, or using new emerging fabrication technologies such as micro injection molding. These former processes involve building the device up layer by layer, involving several material deposition and etch steps. A typical MEMS fabrication technology may have a 5 step process. Due to the limitations of this "traditional IC" manufacturing process MEMS devices are substantially planar, having very low aspect ratios (typically 5 -10 micro meters thick). It is important to note that there are several evolving fabrication techniques that allow higher aspect ratios such as deep x-ray lithography, electrodeposition, and micro injection molding.
MEMS devices are typically fabricated onto a substrate (chip) that may also contain the electronics required to interact with the MEMS device. Due to the small size and mass of the devices, MEMS components can be actuated electrostatically (piezoelectric and bimetallic effects can also be used). The position of MEMS components can also be sensed capacitively. Hence the MEMS electronics include electrostatic drive power supplies, capacitance charge comparators, and signal conditioning circuitry. Connection with the macroscopic world is via wire bonding and encapsulation into familiar BGA, MCM, surface mount, or leaded IC packages.

A common MEMS actuator is the "linear comb drive" (shown above) which consists of rows of interlocking teeth; half of the teeth are attached to a fixed "beam", the other half attach to a movable beam assembly. Both assemblies are electrically insulated. By applying the same polarity voltage to both parts the resultant electrostatic force repels the movable beam away from the fixed. Conversely, by applying opposite polarity the parts are attracted. In this manner the comb drive can be moved "in" or "out" and either DC or AC voltages can be applied. The small size of the parts (low inertial mass) means that the drive has a very fast response time compared to its macroscopic counterpart. The magnitude of electrostatic force is multiplied by the voltage or more commonly the surface area and number of teeth. Commercial comb drives have several thousand teeth, each tooth approximately 10 micro meters long. Drive voltages are CMOS levels.
The linear push / pull motion of a comb drive can be converted into rotational motion by coupling the drive to push rod and pinion on a wheel. In this manner the comb drive can rotate the wheel in the same way a steam engine functions!
The First MEMS Device:
In case you were wondering microsystems have physically been around since the late 1960's. It is generally agreed that the first MEMS device was a gold resonating MOS gate structure. [H.C. Nathanson, et al., The Resonant Gate Transistor, IEEE Trans. Electron Devices, March 1967, vol. 14, no. 3, pp 117-133.]

Schematic of the first MEMS device
Microsystem Analysis Requirements:
Microsystems are inherently multiphysics in nature and thus require a sophisticated coupled physics analysis capability in order to capture actuation and transducer effects accurately. The following analyis features are fundamental requirements for the analysis solution:
• Requires a system of units applicable to small geometric scale.
• Ability to handle unique material properties that are not in the public domain.
• Ability to mesh high aspect ratio device geometry.
• Lumped parameter extraction & reduced order macro modeling for system level simulation.
• Ability to model large field domains associated with electromagnetics and CFD.






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