Micro-electro-mechanical systems


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Micro-electro-mechanical systems (MEMS)

Systems that couple micromechanisms with microelectronics. Such systems are also referred to as microsystems, and the coupling of micromechanisms with microelectronics is also termed micromechatronics. Micromechanics refers to the design and fabrication of micromechanisms that predominantly involve mechanical components with submillimeter dimensions and corresponding tolerances of the order of 1 micrometer or less. The types of systems encompassed by MEMS represent the need for transducers that act between signal and information processing functions, on the one hand, and the mechanical world, on the other. This coupling of a number of engineering areas leads to a highly interdisciplinary field that is commensurately impacting nearly all branches of science and technology in fields such as biology and medicine, telecommunications, automotive engineering, and defense. Ultimately, realization of a “smart” MEMS may be desired for certain applications whereby information processing tasks are integrated with transduction tasks, yielding a device that can autonomously sense and accordingly react to the environment. See Transducer

Motivating factors behind MEMS include greater independence from packaging shape constraints due to decreased device size. In addition, the advantages of repeatable manufacturing processes as well as economic advantages can follow from batch fabrication schemes such as those used in integrated circuit processing, which has formed the basis for MEMS fabrication. Many technical and manufacturing trade-offs, however, come into play in deciding whether an integrated approach is beneficial. In some cases, the device design with the greatest utility is based on a hybrid approach, where mechanical processing and electronic processing are separated until a final packaging step. Two broad categories of devices follow from the transduction need addressed by MEMS: the input transducer or microsensor, and the output transducer or microactuator. See Integrated circuits

Microfabrication technology

The development of process tools and materials for MEMS is the pivotal enabler for integration success. A material is chosen and developed for its mechanical attributes and patterned with a process amenable to co-electronic fabrication. Two basic approaches to patterning a material are used. Subtractive techniques pattern via removal of unwanted material, while additive techniques make use of temporary complementary molds within which the resulting structure conforms. Both approaches use a mask to transfer a pattern to the desired material. For batch processes, this step typically occurs via photolithography and may itself entail several steps. The basic process is to apply a photoresist, a light-sensitive material, and use a photomask to selectively expose the photoresist in the desired pattern. A solvent chemically develops the photoresist-patterned image, which then may be used as a mask for further processing.

Subtractive processing is accomplished via chemical etching. Wet etching occurs in the liquid phase, and dry etching or gas-phase etching may occur in a vapor phase or plasma.

A primary microfabrication technology that has been used for most commercial devices is bulk micromachining, which is the process of removing, or etching, substrate material. The important aspect of precision bulk micromachining is etch directionality. The two limiting cases are isotropic, or directionally insensitive, and anisotropic, or directionally dependent, up to the point of being unidirectional.

An alternative processing approach to bulk microfabrication was driven by the desire to reduce the fraction of the substrate area that had to be devoted to the mechanical components, thereby allowing a larger number of device dies per wafer. The approach, termed surface micromachining (SMM), realizes mechanical structures by depositing and patterning mechanical material layers in conjunction with sacrificial spacer material layers.

Applications

A highly successful device that is fabricated with both bulk and surface micromachining is the integrated pressure transducer. The process sequence uses surface micromachining techniques to form a polysilicon-plate-covered cavity. Application areas include air pressure sensing in automobile engines, environmental monitoring, and blood pressure sensing. Similar processing has resulted in the integration of surface-micromachined polysilicon inertial reference proof masses with microelectronic processing, yielding single-chip force-feedback accelerometers. See Accelerometer

The use of surface micromachining technology to implement microactuators has resulted in steerable micromirror arrays with as many as 1024 × 768 pixels on a chip. These arrays have revolutionized digital display technology. Further electrostatic microactuator designs are possible and may be extremely intricate, such as a torsional ratcheting actuator fabricated with five polysilicon levels (Fig. 1). These types of devices are suited for a variety of micropositioning applications. Processing based on deep-x-ray lithography has been used to produce precision magnetic microactuators. One such microactuator directly switches a single-mode optical fiber in a 1 × 2 switch configuration (Fig. 2).

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