Micro-electromechanical systems, commonly referred to as MEMS, are one of the most promising forms of technology in the scientific world today. There are many process technology methods for producing MEMS, as well as a wide range of applications in which the devices can be used. In this article, we look at the different types of MEMS and their applications.
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An Introduction to MEMS
MEMS are a combination of mechanical and electronic components which are integrated to form a complex system. In their most fundamental sense, they are a combination of microactuators, microsensors, mechanical microstructures and microelectronics - all of which are integrated onto a single structural substrate. Their relative sizes vary depending on the integration technique used to produce the system, but they typically sit within the range of a few micrometers up to the millimeter scale. The key feature of MEMS is that they can sense, control and actuate at the micro-level, but the actual effects of the device are realized at the macro-level.
MEMS can be made from a wide range of materials and, as such, there are many ways in which MEMS can be fabricated. Unlike larger analogous components that perform the same job (which are created through bulk machining methods), MEMS components are produced through various microfabrication methods. The most common techniques used in the creation of MEMS are lithographic patterning methods, wet and dry etching methods, fusion bonding, physical vapour deposition (PVD), chemical vapour deposition (CVD), fusion bonding, laser micromachining, passivation and encapsulation methods.
Types of MEMS
Silicon MEMS
Silicon is the most widely used material in electronic circuits and devices, so it was only natural that silicon became one of the first materials to be used in MEMS devices. Because they have been used in circuits for many years, there are no issues in fabricating the MEMS devices using top-down fabrication methods, nor are there any issues when integrating them with other components or circuits. In many respects, they were historically the safest material and are still one of the most widely used materials in MEMS devices today.
Metal MEMS
Metals are less commonly used than silicon in MEMS devices, mainly due to the fact that they possess fewer mechanical properties. However, there are times when a high degree of mechanical strength or deformation is not required, and metal components can be created using simpler bottom-up deposition methods, rather than more complex top-down methods. When metal components are used within their specific limits, they can produce very reliable components. Some of the metals used within MEMS systems include gold, nickel, aluminium, copper, chromium, titanium, tungsten, platinum, and silver, and the methods of choice for creating these devices are usually electroplating, evaporation and sputtering.
Polymer MEMS
Polymer MEMS have started to become an increasingly popular option for many years now, and this has been brought about by the many advantages that can be exploited from their flexible nature - to deform, conform and be manipulated into specific geometries. There are also many different polymers that can be used with MEMS devices, which expands the capabilities of this area, especially when they can be produced in much greater volumes than silicon and other MEMS materials. However, because of the many physical requirements (for both the fabrication and structural aspects of the device) that are required with MEMS systems, only certain polymers can be used—with the most common types being SU-8, PDMS, Parylene and Polyimides, as well as composites composed of a polymeric matrix.
Polymers are widely used to improve the design, fabrication and packaging processes of MEMS devices. The elastic properties of polymers are more forgiving and more versatile than other common MEMS structural materials (such as silicon), and this is directly related to the mechanical performance as a function of Young’s modulus.
Internal stresses can cause serious deformation to MEMS devices. The flexibility and resistance of polymers to many environmental processes enables them to combat some of stresses that can arise in other MEMS devices. Most polymers are also naturally strong, with a high tensile strength. Once this is coupled with their flexibility, the produced devices are inherently strong and are resistant to breakage when compared to those of a rigid and weaker nature. Finally, polymers are highly resistant to corrosion, which enables MEMS devices (constructed from this material) to minimize wear and fatigue. This is an especially useful set of properties for electromechanical environments, where high temperatures and degradation are commonplace.
Applications of MEMS
In terms of their real-world use (and not just their interest at an academic level), MEMS have found ample uses and commercial potential in tech markets that combine microelectronics and micromachining technologies. The MEMS field itself is a complex area, operating in many large-scale industries and encompassing elements of mechanical engineering, materials science, electrical engineering, chemistry and chemical engineering, among other areas of applied science.
In terms of specific applications, they are often used as transducers (i.e. they change one form of energy into another), and are known to be used in heat exchangers, inkjet printer heads, micro-mirror arrays, projectors, high pressure sensors, infrared detectors, microphones, optical switching technologies, displays, energy harvesting applications and in lab-on-a-chip devices.
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