Micro Electro-Mechanical Systems

Micro Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate using microfabrication techniques. MEMS are a hot area of research because they integrate sensing, analyzing and responding on the same silicon substrate hence promising realization of complete systems-on-a-chip. As MEMS are manufactured using batch fabrication techniques similar to IC technology, MEMS are expected to deliver high functionality at low prices.

Current systems are limited by the capability of sensors and actuators, as these are bulkier and less reliable than the microelectronic circuit. In a MEM system the sensors act as the ‘eyes’ and gather data about the environment. The microelectronic circuit, which is the ‘brain’, processes the data and accordingly controls the mechanical systems, the ‘arms’ of the MEMS, to modify the environment suitably. The electronics on the MEMS are manufactured using IC techniques while micro machining techniques are used to produce the mechanical and electromechanical parts.
MEMS Fabrication:
There are number of methods to fabricate MEMS like silicon surface micromachining, silicon bulk machining, electro discharge machining, LIGA (in German, Lithographie, Galvanoformung(Electro Plating), Abformung(Injection Moulding)) .Only silicon surface micromachining is discussed here.

Silicon Surface Micromachining
Silicon surface micromachining uses the same equipment and processes as the electronics semiconductor industry. There are three basic building blocks in this technology, which are the ability to deposit thin films of material on a substrate, to apply a patterned mask on top of the films by photolithographic imaging, and to etch the films selectively to the mask. A MEMS process is usually a structured sequence of these operations to form actual devices.

1. Deposition Processes: One of the basic building blocks in MEMS processing is the ability to deposit thin films of material. This is achieved either through the processes like PVD or CVD.
2. Lithography: Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive material by selective exposure to a radiation source such as light. In lithography for micromachining, the photosensitive material used is typically a photoresist. When resist is exposed to a radiation source of a specific a wavelength, the chemical resistance of the resist to developer solution changes.

3. Etching: If the resist is placed in a developer solution after selective exposure to a light source, it will etch away one of the two regions (exposed or unexposed). Then another layer of material is deposited and the first layer selectively etched away as shown in the figure resulting in the required MEMS.
Comb drives actuators and electrostatic motors can be fabricated using this technique.

Fig: The processes involved in Silicon Micromachining technique.

The LIGA process is used to make MEMS with a higher accuracy. Actuators in MEMS are made using either piezoelectric effect or shape memory alloys.

Fig: Actuator using piezoelectric effect
Application of MEMS:
As MEMS are smart the applications one can conjure for them is limitless. On one hand scientist in University of California, Berkeley are developing dust particles which gather information about the weather while on the other MEMS are already being used in the automotive industry as accelerometers for triggering airbags. Some of the applications of MEMS in the industry are as follows.

1. Pressure, temperature and Chemical Sensors: MEMS pressure sensors are made using piezoelectric strain gauges placed on a silicon substrate. Biomedical uses of these sensors are to measure the blood pressure inside arteries while aerospace uses the same for measuring airspeed and altitude information.

2. Light reflectors: Micro mirrors are an array of small mirrors whose alignment direction can be varied by application of a voltage to the device. These reflectors are finding uses in projection devices, optical switches and wavelength selective filters.
3. Optical Switches: Based on micro mirrors, MEMS-based switches route light from one fiber to another. Such an approach enables a truly photonic (completely light-based) network of voice and data traffic, since switching no longer requires conversion of light signals into digital electronic signals and then back to optical.
4. Accelerometers (for airbags, pacemakers and games):The conventional approach for triggering airbags uses several bulky accelerometers made of discrete components mounted in the front of the car with separate electronics near the airbag; this approach costs over $50 per automobile. MEMS technology has made it possible to integrate the accelerometer and electronics onto a single silicon chip at a cost of $5 to $10. These MEMS accelerometers are much smaller, more functional, lighter, more reliable, and are produced for a fraction of the cost of the conventional accelerometer elements. Accelerometers also come handy in inertial navigation and guidance systems.
Fig. The design of a MEMS accelerometer
5. Microactuators for data storage and read/write heads: MEMS based accelerometers are used to provide feedback to the drive heads to control the accuracy of the motion of read/write heads over the hard disks.

6. Improving Communications components: MEMS technology can help decrease size and improve quality of highly demanding wireless communications hardware as shown in the figure below.

7. Applications in Aerospace: Pressure sensing of fuel, oil, transmission system, hydraulics, Microgyroscopes for navigation and stability control, Airspeed and Altitude measurements.
To summarize, any process which requires sensing, analyzing and reacting to the data are good candidates for MEMS implementation.