Micro-Electro-Mechanical Systems (MEMS)

Micro-Electro-Mechanical Systems, or MEMS, are devices formed by integrating mechanical elements, sensors, actuators, and electronics onto a common silicon substrate using various processes of semiconductor wafer fabrication (for the electronic components) and micromachining (for the micromechanical components). This fusion of semiconductor and micromachining capabilities makes MEMS an enabling technology for a whole new set of exciting products, while revolutionizing those that already exist.

MEMS brings the concept of complete 'system-on-a-chip' (SOC) a step higher, by allowing a microelectronic device to: 1) 'feel' its surroundings in ways never before possible through the help of sensors with moving parts; and 2) physically affect objects around it through microscopic actuators, motors, hinges, linkages, pivots, gears, and the like. Indeed, MEMS almost promises to deliver science fiction-inspired microscopic robots of the future that can think, sense, and move inside the human body.

For now, however, MEMS applications are more widely deployed in systems that are commonly encountered in our daily lives, such as: 1) automobiles' airbag deployment systems, tire pressure sensors, and suspension control mechanisms; 2) motion input devices for gaming and training consoles; 3) blood pressure sensors; 4) inkjet printers' ink deposition systems; 5) optical switching systems for communications equipment; and 6) micromirror arrays for projection TV's.

Figure 1. Examples of MEMS Structures

Source of photos: www.memx.com

MEMS microfabrication techniques that are used to complement the wafer fabrication techniques come in various forms, including: 1) silicon surface micromachining; 2) silicon bulk micromachining; 3) electrical discharge machining (EDM); and 4) the LIGA (Lithographie, Galvanoformung, Abformung) technology, which stands for "lithography, plating, molding".

Silicon surface machining, which builds the micromechanical devices on the surface of a silicon wafer, is widely used in the semiconductor industry, and for obvious reasons. It integrates well into microelectronic products, since it employs basically the same techniques used in conventional wafer fabrication. In this technique, thin layers of structural and sacrificial materials are deposited over the wafer surface in precise patterns. The micromechanical system is completed and left on the surface upon removal of the sacrificial material at the end of the process.

As in wafer fabrication, silicon dioxide is often used as sacrificial material in surface micromachining. On the other hand, structural features are often built using polysilicon layers. With the sacrificial layers serving as high-resolution masks to prevent the structural material from being deposited in areas where it shouldn't be, the shapes of the mechanical structures may be defined with high precision. The sacrificial layers are then removed, usually by etching with buffered hydrofluoric acid (HF), leaving the mechanical features intact on the silicon surface.

Silicon bulk machining refers to the formation of micromechanical systems by etching them out of bulk silicon, allowing structures with greater heights to be built. It employs either etchants that stop on the crystallographic planes of the silicon wafer or etchants that act isotropically (i.e., active in all directions) to generate mechanical parts. The resulting systems can then be integrated into other structures by wafer bonding. Micromechanical structures that have been fabricated through silicon bulk machining include mirrors and accelerometer devices.

Electrical discharge machining (EDM), which was developed by Matsushita, is basically just an extension of conventional machine shop technology to fabrication of parts in sub-millimeter sizes. It is, in fact, compatible with machine shop production techniques. A typical EDM process employs high-frequency electrical sparks from a graphite or metal electrode to disintegrate electrically conductive materials such as hardened steel or carbide. The electrode and the workpiece are separated by a small gap (which is about 10-100 microns) and immersed in a dielectric fluid as this occurs.

LIGA processes combine IC lithography with electroplating and molding techniques to obtain depth. Patterns are formed on a substrate and then plated with an electrodeposited metal such as nickel to create three-dimensional molds. These molds can be used as the final products themselves or may be injected with various materials. LIGA has two main advantages: 1) it allows the use of non-silicon and non-metal materials such as plastic; and 2) it allows the fabrication of devices with very high aspect ratios.

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