Overview
The Micro Electro Mechanical Systems technology (aka MEMS) allows to integrate on the same silicon substrate both electronic circuits and optical-mechanical devices, adopting semiconductor fabrication technologies similar to those used to make integrated circuits. The size of a MEMS device usually ranges from some micrometers to one millimeter, whereas the single components it is made up of may be from just 1 to 100 micrometers in size. MEMS is a challenging and promising technology which, merging together silicon-based and micromachining technologies, makes it possible the realization of smart and integrated system-on-a-chip devices. A MEMS device can be thought of a “brain” (an integrated IC) and a set of “arms” and “eyes” through which it monitors and controls the environment. Micro sensors (the “eyes”) gather information from the environment measuring position, movement, temperature, pressure, magnetic field, optical and chemical phenomena. That information can then be processed by the “brain” which acts on the environment by means of the “arms”, that are basically micro actuators. Figure 1 shows a multiple gears unit with a chain in the foreground. The white bar at the top of the picture (500 micrometers in length) gives you an idea of the actual object size.

This technology is literally revolutionazing the way several kinds of products are made. Categories already involved in this change are: accelerometers, gyroscopes, pressure sensors. MEMS technology is gradually replacing the standard products because of its several advantages: less power consumption, less weight and smaller size, better performance, lower costs and higher performance and reliability. Another factor which is focusing the interest on MEMS technology is due to its potential integration with the nanotechnology world, a sector with is experiencing an amazing expansion in the last few year thanks also to relevant public investments.

Manufacturing methods
Most of the MEMS fabrication methods are adopted from standard IC technology. The most common techniques are: bulk micromachining, surface micromachining, and LIGA (Roentgen LItography GAlvanic Abformung). In bulk micromachining, a 3D micro-mechanical structure is built directly on the silicon wafer by selectively removing portions of the substrate. Surface micromachining is based on the deposition of layers on the substrate, and on the subsequent definition of the micro-mechanical structure by means of photolithographic techniques. LIGA is based on the following three phases: lithography, deposition, and molding. This technique allows to use materials different from silicon, such as polymers and metals, and to obtain structures with a very high aspect ratio.

Manufacturing processes
The fabrication method is structured on the following three processes: deposition, etching, and lithography. Deposition consists in the ability to deposit thin films of material on the substrate (size ranges from few nanometers to 100 micrometers); deposition can be obtained either by chemical reactions (chemical vapor deposition, electrodeposition, epitaxy, thermal oxidation) or by physical reactions (physical vapor deposition and casting). Etching is a process through which selected portions of the thin films or the substrate itself are removed in order to obtain the desired MEMS 3D structure. There are two kinds of etching: wet etching and dry etching. With the first one the material is dissolved dipping it into a chemical solution, whereas with the second one the material is dissolved using reactive ions or a vapor phase etchant. There is a special kind of dry etching called Deep Reactive Ion Etching (DRIE) that is rapidly increasing its popularity. It was developed in the ’90 in Germany by the Bosch company, it is also called “Bosch process”, and is based on the alternation of two different gas compositions in the reactor. This process can easily achieve an aspect ratio of 50 to 1, even though it is more expensive than the wet etching. Lithography is the main process for pattern definition in micromachining. The lithography process applied to the MEMS technology consists in the selective exposure of a photosensitive material deposited on the substrate to a source of radiation at a given wavelength, such as light. Exposure regions are selected by means of a pre-formatted mask. A typical photosensitive material used for MEMS is the photoresist.

Applications
MEMS devices already produced or in a deep development phase can be grouped in the following categories:

sensors and actuators
radio frequency MEMS
optical MEMS
bio MEMS

Sensors and actuators
The most common types of MEMS sensors are:

pressure sensors
accelerometers and gyroscopes
weight/strength sensors
speedometers

MEMS pressure sensors were initially introduced in the ’60 for military and aerospace applications. Nowadays, they are playing an important role especially in the automotive and biomedical fields (blood pressure sensors).
Accelerometers and gyroscopes are mainly used for airbag systems, with millions of units produced in the last few years, but recently they are being heavily used also for console game controllers.
Speedometers were introduced in the ’90 and are used in the automotive market (stability control, tire pressure sensor, GPS) and in electronics products (stability control for cameras, GPS system for mobile phones). Weight sensors are used for home scales.
Actuators are used to generate movement or strength able to move other MEMS components. They can be grouped into electrostatic and thermal actuators. In the electrostatic actuators an electrical field is applied between a fixed and a moveable structure, usually interleaved to create a “comb” figure. Figure 2 shows an electro-thermally actuated micro gripper.

RF MEMS
RF MEMS are mainly used in mobile and cordless phones and in GPS receivers. They provide an outstanding performance in terms of: very small size, wide bandwidth, low price and an extremely high signal noise ratio, which is a must for radio frequency devices. This kind of MEMS is rapidly growing and replacing the corresponding solutions in solid state technology. In Figure 3 is shown a copper spiral inductor integrated on a RF MEMS device.

Optical MEMS
Optical MEMS are essentially adopted to drive, amplify or attenuate, and reflect an optical signal at a given wavelength. They are especially used in switching devices and in optical modules for fiber optics transmission equipments. In common optical switches and cross connection devices there is often the need to modify the optical path of light; that is achieved by micro machined mirrors built with MEMS technology. A MEMS micro mirror is visible in Figure 4.

Micro fluid and bio MEMS
Micro fluid MEMS are devices designed to work with and operate on fluids at a microscopic level. Typical applications of micro fluid MEMS are: valves, pumps, injectors (lots of them used in ink-jet printers).
Bio MEMS are similar to micro fluid MEMS, with the difference that they are primarily designed to handle biological fluids such as blood, for instance. Bio MEMS are mainly intended for medical and health treatment applications, such as Lab-on-a-chip, and Micro Total Analysis.

Links
http://www.mems.sandia.gov/
http://www.memsnet.org/

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