MEMS

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MEMS

Microelectromechanical Systems (MEMS) is the technology of the very small, and merges at the nanoscale into "Nanoelectromechanical Systems" (NEMS) and Nanotechnology. In Europe, MEMS are often referred to as Micro Systems Technology (MST). It should not be confused with the hypothetical vision of Molecular nanotechnology or Molecular Electronics. ==of a metre) to a millimetre (thousandth of a metre). At these size scales, a human's intuitive sense of physics do not always hold true. Due to MEMS' large surface area to volume ratio, surface effects such as electrostatics and wetting dominate volume effects such as inertia or thermal mass. They are fabricated using modified silicon fabrication technology (used to make electronics), molding and plating, wet etching (KOH, TMAH) and dry etching (RIE and DRIE), electro discharge machining (EDM), and other technologies capable of manufacturing very small devices. MEMS sometimes go by the names micromechanics, micro machines, or micro system technology (MST).

Companies with strong MEMS programs come in many sizes. The larger firms specialize in manufacturing high volume inexpensive components or packaged solutions for end markets such as automobiles, biomedical, and electronics. The successful small firms provide value in innovative solutions and absorb the expense of custom fabrication with high sales margins. In addition, both large and small companies work in R&D to explore MEMS technology. Complexity and performance of advanced MEMS based sensors are described by different MEMS sensor generations.

Common applications include:

inkjet printers, which use piezoelectrics or bubble ejection to deposit ink on paper.
accelerometers in modern cars for airbag deployment in collisions.
MEMS gyroscopes used in modern cars and other applications to detect yaw; e.g. to deploy a roll over bar or trigger dynamic stability control.
pressure sensors e.g. car tire pressure sensors, and disposable blood pressure sensors.
Displays e.g the DMD chip in a projector based on DLP technology has on its surface several hundred thousand micromirrors.
Optical switching technology which is used for switching technology for data communications, and is part of the emerging technology of smartdust.

Finite element analysis is an important part of MEMS design.

MEMS Materials

MEMS technology can be implemented using a number of different materials and manufacturing techniques; the choice of which will depend on the device being created and the market sector in which it has to operate.

Silicon

Silicon is the material used to create almost all integrated circuits used in consumer electronics in the modern world. The economies of scale, ready availability of highly accurate processing and ability to incorporate electronic functionality make silicon attractive for a wide variety of MEMS applications. Silicon also has significant advantages engendered through its material properties. In single crystal form, silicon is an almost perfect Hookean material, meaning that when it is flexed there is virtually no hysteresis and hence almost no energy dissipation. As well as making for highly repeatable motion, this also makes silicon very reliable as it suffers very little fatigue and can have service lifetimes in the range of billions to trillions of cycles without breaking. The basic techniques for producing all silicon based MEMS devices are deposition of material layers, patterning of these layers by lithography and then etching to produce the required shapes.

Polymers

Even though the electronics industry provides an economy of scale for the silicon industry, crystaline silicon is still a complex and relatively expensive material to produce. Polymers on the other hand can be produced in huge volumes, with a great variety of material characteristics. MEMS devices can be made from polymers by processes such as injection moulding, embossing or stereolithography and are especially well suited to microfluidic applications such as disposable blood testing cartridges.

Metals

Metals can also be used to create MEMS elements. While metals do not have some of the advantages displayed by silicon in terms of mechanical properties, when used within their limitations, metals can exhibit very high degrees of reliability.

Metals can be deposited by electroplating, evaporation, and sputtering processes.

Commonly used metals include Gold, Nickel, Aluminum, Chromium, Titanium, Tungsten, platinum, and silver.

MEMS Processes

Deposition Processes

One of the basic building blocks in MEMS processing is the ability to deposit thin films of material. In this text we assume a thin film to have a thickness anywhere between a few nanometer to about 100 micrometer. Commonly used deposition processes are: Electroplating, Sputtering, Physical Vapour Deposition (PVD) and Chemical Vapour Deposition (CVD).

Photolithography

Main article: Photolithography

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. A photosensitive material is a material that experiences a change in its physical properties when exposed to a radiation source. If we selectively expose a photosensitive material to radiation (e.g. by masking some of the radiation) the pattern of the radiation on the material is transferred to the material exposed, as the properties of the exposed and unexposed regions differs.

This exposed region can then be removed or treated providing a mask for the underlying substrate. Photolithography is typically used with metal deposition, wet and dry etching.

Etching Processes

There are two basic categories of etching processes: wet and dry etching. In the former, the material is dissolved when immersed in a chemical solution. In the latter, the material is sputtered or dissolved using reactive ions or a vapor phase etchant.

Wet Etching

Main article: Wet etching

This is the simplest etching technology. All it requires is a container with a liquid solution that will dissolve the material in question. Unfortunately, there are complications since usually a mask is desired to selectively etch the material. One must find a mask that will not dissolve or at least etches much slower than the material to be patterned. Secondly, some single crystal materials, such as silicon, exhibit anisotropic etching in certain chemicals. Anisotropic etching in contrast to isotropic etching means different etch rates in different directions in the material. The classic example of this is the <111> crystal plane sidewalls that appear when etching a hole in a <100> silicon wafer in a chemical such as potassium hydroxide (KOH). The result is a pyramid shaped hole instead of a hole with rounded sidewalls with an isotropic etchant. The principle of anisotropic and isotropic wet etching is illustrated in the figure below.

RIE etching

Main article: Reactive ion etching

In reactive ion etching (RIE), the substrate is placed inside a reactor in which several gases are introduced. A plasma is struck in the gas mixture using an RF power source, breaking the gas molecules into ions. The ions are accelerated towards, and reacts at, the surface of the material being etched, forming another gaseous material. This is known as the chemical part of reactive ion etching. There is also a physical part which is similar in nature to the sputtering deposition process. If the ions have high enough energy, they can knock atoms out of the material to be etched without a chemical reaction. It is a very complex task to develop dry etch processes that balance chemical and physical etching, since there are many parameters to adjust. By changing the balance it is possible to influence the anisotropy of the etching, since the chemical part is isotropic and the physical part highly anisotropic the combination can form sidewalls that have shapes from rounded to vertical. A schematic of a typical reactive ion etching system is shown in the figure below.

DRIE etching

Main article: Deep reactive ion etching

A special subclass of RIE which continues to grow rapidly in popularity is deep RIE (DRIE). In this process, etch depths of hundreds of micrometres can be achieved with almost vertical sidewalls. The primary technology is based on the so-called "Bosch process", named after the German company Robert Bosch which filed the original patent, where two different gas compositions are alternated in the reactor. The first gas composition creates a polymer on the surface of the substrate, and the second gas composition etches the substrate. The polymer is immediately sputtered away by the physical part of the etching, but only on the horizontal surfaces and not the sidewalls. Since the polymer only dissolves very slowly in the chemical part of the etching, it builds up on the sidewalls and protects them from etching. As a result, etching aspect ratios of 50 to 1 can be achieved. The process can easily be used to etch completely through a silicon substrate, and etch rates are 3-4 times higher than wet etching.

Bulk micromachining

Main article: Bulk micromachining

Bulk micromachining is similar to deep etching but uses a different process to remove silicon. Bulk micromachining typically uses alkaline liquid solvents, such as potassium hydroxide, to dissolve silicon which has been left exposed by the photolithography masking step. These alkali solvents dissolve the silicon in a highly anisotropic way, with some crystallographic orientations dissolving up to 1000 times faster than others. Such an approach is often used with very specific crystallographic orientations in the raw silicon to produce v-shaped grooves. The surface of these grooves can be atomically smooth if the etch is carried out correctly with dimensions and angles being extremely accurate.

Metals are also often used as masks for dry and wet etching other materials depending on the selectivity of the metal to the etchant.

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