Microelectromechanical Systems MEMS

Microelectromechanical systems (MEMS) have already found significant applications in sectors that include, but are not limited to: automotive industry, aircraft industry, chemical industry, pharmaceuticals, manufacturing, defence, and environmental monitoring. The relative merit for MEM systems lies in the fact that these components are fabricated by batch manufacturing methods similar to microelectronics techniques, which fulfills the added advantage of miniaturization, performance and integrability. The topical areas under MEMS are micromachining methods, microsensors and actuators, magnetic MEMS, RF MEMS, microfluidics, BioMEMS and MOEMS. The progress in microfabrication technologies is transforming the field of solid-state into MEMS. Micromachining is a process for the fabrication of MEMS devices and systems. Various energy transduction principles include thermal, magnetic, optical, electrical and mechanical. These are employed in designing the microsensors and actuators. Radio Frequency (RF) MEMS devices are mostly used in the field of wireless communication. Microfluidic MEMS devices handle and control small volumes of fluids in the order of nano and pico liter volumes. One popular application is a micronozzle for use in printing applications. MEMS technology has applications in the chemical industry, which gives rise to BioMEMS products. Surgical instruments, artificial organs, genomics, and drug discovery systems are based on BioMEMS products.

The miniaturised systems require less reagent, resulting in faster and more accurate systems. MEMS devices have better response times, faster analysis and diagnosis capabilities, better statistical results, and improved automation possibilities with a decreased risk and cost. Since most of the physical phenomenon and activations are to be measured and controlled precisely in a timely predictive manner so as to overcome realtime limitations, miniaturised components will have added advantages because of the inherent temporal behavior they possess. Moreover, prognostic measures in terms of sensor and actuator validation can be achieved through a system-on-a-chip (SOC) design approach. MEMS devices are useful for controlling micro mechanisms such as micromanipulators, micro-handling equipment, microgrippers, microrobot, and others, which are primarily used for clinical, industrial and space applications.

1.3.1 An Example: Microphenomenon in Electrophotography

Electrophotography can by used for a broad range of applications. These include:

• Manufacturing of PCB

• Creating images on a wide variety of receivers

• Thin coating on pharmaceutical tablets and capsules

Electrophotography produces documents and images. The ability to do so requires that micrometer-size marking, toner, and particles be precisely placed on a receiver by adopting micro level technology. In order to produce high quality electrophotographic images, it is necessary to carefully control the forces acting on the toner particles. These forces are predominantly either electrostatic or electrodynamic in nature. The precision in color electrophotography process is extraordinary. Color electrophotography is an advanced technique. It requires that the separations comprising the subtractive primary images be precisely superimposed in order to create a sharp image with excellent color balance.

1.4 Microelectronics Fabrication Methods

One of the major inventions in the last century is microelectronics, called micro devices. Micro devices can be integrated circuits, which are fabricated in submicron dimensions and form the basis of all electronic products. Microelectronics design entails the accommodation of essential attributes of modern manufacturing. Fabrication technology, starting from computer assisted off-line design to real fabrication, deals with the processes for producing electronic circuits, solid structures, printing circuits as well as various electronic components, sub-systems and systems of subminiature size.

The design of an IC with millions of transistors and even more interconnections is not a trivial task. Before the real design is manufactured, the circuit is prepared and tested by using EDA (electronic design automation) tools. These tools help in synthesizing and simulating the behavior of the desired circuit by arranging the placement of transistors and interconnections within the chip area. These computer-assisted tools can also verify and validate all defects and conditions, respectively. The technology has been driven by the demands of the computer industry, space technology, the car industry and telecommunications.

The first step in fabrication is always the preparation of a set of photographic masks. The mask represents the features of the various elements and layers of the chip to be manufactured. This procedure is repeated several times to replicate the circuit. The mask appears on the surface of a thin silicon crystal wafer. A single wafer can accommodate several identical chips. Hence, the IC fabrication process is a batch-processing scheme. The preparation of masks can be carried out by the use of a computer-controlled electron beam to expose the photographic mask material in accordance with the desired configuration. The information is supplied to the computer in terms of a design data file.

Then three important fabrication sequences are followed on the wafer surface. These are photography followed by chemical, and thermal operations. This phase is called masking. The mask features are transferred to the wafer by exposing a light-sensitive photoresist coating through the transparent areas of the mask. The material areas of the wafer unprotected by the hardened photoresist are then removed by etching.

Etching techniques are characterised by their selectivity and degree of anisotropy. Etching can be either physical or chemical, or a combination of both. In order to develop active circuit elements such as transistors, n-type and p-type impurities are doped. Two commonly used doping methods are diffusion and ion implantation. Then a thin aluminum layer is deposited on the uppermost layers of the chip in order to allow metal to contact the device elements. The aluminum deposition is often achieved by using the chemical vapor deposition (CVD) method.

1.4.1 Bulk Micromachining

The term micromachining refers to the fabrication of micromechanical structures with the aid of etching techniques to remove part of the substrate or a thin film (Petersen, 1982). Silicon has excellent mechanical properties, making it an ideal material for machining. The fabrication processes fall into the two general categories of bulk micromachining and surface micromachining.

In general, the process uses the bulk material or substrate to form microstructures by etching directly into the bulk material. Bulk micromachining refers to etching through the wafer from the backside in order to form the desired structures. The structures are formed by wet chemical etching or by reactive ion etching (RIE). Usually, suspended micro structure s are fabricated using wet chemical micromachining. The advantage of bulk micromachining and chemical etching is that substrate materials such as quartz and single crystal silicon are readily available and reasonably high aspect-ratio structures can be fabricated. It is also compatible to IC technologies, so electronics can be easily integrated. Disadvantages of bulk micromachining include pattern and structure sensitivity and pattern distortion due to different selective etch rates on different crystallographic planes (Bean 1978; Kern 1990; Danel and Delapierre 1991). Further, since both the front side and backside are used for processing, severe limits and constraints are encountered on the minimum feature size and minimum feature spacing.

1.4.2 Surface Micromachining

Surface micromachining is a process of fabrication of MEMS structures out of deposited thin films. The process is also employed for IC fabrication. It involves the formation of mechanical structures in thin films formed on the surface of the wafer. The thin film is primarily composed of three layers: low-pressure chemical-vapor-deposition polycrystalline silicon, silicon nitride, and silicon dioxides. They are deposited in sequence and subsequently selectively removed to build up a three-dimensional mechanical structure integrated with the electronics. The structure is essentially freed from the planar substrate. The process is very complex in the sense that it requires serious attention during the process, as the property of material significantly varies at the microstructure level. In particular, the following issues are dealt with cautiously (James 1998):

• Basic understanding and control of the material properties of thin films

• Fabrication features for hinged structures and high-aspect ratio devices

• Releasing method for the microstructure

• Packaging methods

Micromachining, using doped or undoped polysilicon as the structural material and silicon dioxide as the sacrificial material, is the most frequently used. Silicon nitride is used as an insulator. Hydrofluoric acid is used to dissolve the sacrificial oxide during release. Primarily, the fabrication process involves the following steps:

• Substrate passivation and interconnection

• Sacrificial layer deposition and patterning

• Structural polysilicon deposition and doping

• Microstructure release, rinse, and dry

Some of the examples of polysilicon micromechanical devices are flexible suspensions, gear trains, turbines, cranks, tweezers, and linkages, which have already been fabricated on silicon. Although bulk micromachining is considered to be the older technology, the two developments (bulk and surface) ran parallel (French 1998).

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