Micro-electro-mechanical systems (MEMS) is the technology of the fabrication and operation of microscopic devices, usually with moving parts. MEMS components are typically sized between 1 and 100 micrometres. Most of them consist of an IC chip (microprocessor) that computes data and other components (micro-sensors) that read, translate, and transmit data from external inputs. MEMS can sense and actuate on a micro-scale and produce output on a macro-scale.
Forces produced by ambient electrostatic charges, surface tension and viscosity, take on great importance in the operation and fabrication of MEMS because of their large surface area-to-volume ratio.
The materials used for MEMS have different mechanical properties when they are evaluated at micro proportions compared to their properties in bulk. The production of MEMS became practical once the fabrication techniques normally used in electronics became adapted to the manufacture of small micro-devices.
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The history of MEMS can be summarised in milestones, as described below [1][2].
1954. The Piezoresistive Effect in silicon and germanium was first discovered by C. S. Smith. It is the change in the electrical resistivity of a material when it has undergone mechanical strain. It was found to be greater in semiconductors than in metals.
1958. Jack Kilby made the first integrated circuit on a germanium chip. Robert Noyce built the first monolithic integrated circuit on a silicon chip a year later.
1964. Harvey Nathanson batch produced the first MEMS device, the Resonant Gate Field Effect Transistor (RGT), which had an approximate length of 1mm. This was also the birth of surface micro-machining.
1971. The first microprocessor was invented by Intel, which paved the way for personal computing.
1979. MEMS technology was used to create the first micro-machined inkjet nozzles (called Thermal Inkjet Technology, TIJ) by Hewlett Packard.
1982. The LIGA process for manufacturing microstructures was created by Karlsruhe Nuclear Research Centre, Germany. LIGA is a German acronym that stands for X-ray lithography, Electroplating, and Moulding.
1986. IBM invented a micro-device called the atomic force microscope (AFM), which is an extremely high-resolution scanning probe microscope that is used to measure the morphology of silicon wafer surfaces.
1993. The first commercially produced surface micromachined accelerometers (Analog Devices, ADXL50) were sold and used in the automotive industry for airbag deployment technology.
1994. Bosch developed and patented Deep Reactive Ion Etching (DRIE), an anisotropic etching process used to make microscopic holes and trenches in micro-devices.
1995. BioMEMS (MEMS technology applied to medical practice) started to develop rapidly.
The application of MEMS technology has been proliferating in so many fields. Below are some of the most common applications detailing their discovery and evolution until their complete commercialisation.
Table 1. Commercialisation of selected MEMS devices [3]
| Product | Discovery | Evolution | Cost Reduction/ Application Expansion | Full Commercialisation |
| Pressure sensors | 1954-1960 | 1960-1975 | 1975-1990 | 1990-present |
| Accelerometers | 1974-1985 | 1985-1990 | 1990-1998 | 1998 |
| Gas sensors | 1986-1994 | 1994-1998 | 1998-2005 | 2005 |
| Valves | 1980-1988 | 1988-1996 | 1996-2002 | 2002 |
| Nozzles | 1972-1984 | 1984-1990 | 1990-1998 | 1998 |
| Photonics/displays | 1980-1986 | 1986-1998 | 1998-2004 | 2004 |
| Bio/Chemical sensors | 1980-1994 | 1994-1999 | 1999-2004 | 2004 |
| RF switches | 1994-1998 | 1998-2001 | 2001-2005 | 2005 |
| Rate (rotation) sensors | 1982-1990 | 1990-1996 | 1996-2002 | 2002 |
| Micro relays | 1977-1982 | 1993-1998 | 1998-2006 | 2006 |
The material used in MEMS technology is predominantly silicon, but thin films made from other materials such as germanium and gallium arsenide are also used. Silicon materials are not considered typically to have properties suited for mechanical applications, and so data regarding their properties are not as extensively studied. There is a need to investigate their properties to adapt them for a wide variety of applications.
Furthermore, materials behave significantly differently in the micro proportions in which they are required to be used in MEMS, and so property data that may be available for bulk materials may need to be re-evaluated for MEMS applications.
There are three major property categorisations that are of concern in MEMS fabrication; elastic, inelastic and strength properties [4]. Other properties, such as thermal, electrical, chemical and optical properties are more dependent on the specific applications the MEMS device is used for. Some commonly regarded properties include dielectric strength, electrical resistivity, thermal conductivity, coefficient of thermal expansion, chemical resistance, and transparency.
Elastic properties are crucial to the performance of a MEMS device and are governed mainly by two parameters; Young’s modulus and Poisson’s ratio. Young’s modulus indicates a material’s stiffness while the Poisson’s ratio describes a material’s expansion in a perpendicular direction to the direction of compression. Both these parameters can be measured through load-deflection techniques.
Plastic deformation occurs when a material deforms under stress without returning to its original shape. It is, thus, vital to study a MEMS material’s behaviour during inelastic deformation. Fatigue is also an important property as it indicates how a MEMS device will perform under constant stress or many cycles of stress. Fatigue causes plastic deformation and in turn, causes failure or a decrease in the performance of a MEMS device.
There are several critical strength-related properties in MEMS technology, such as tensile strength, fracture strength, flexural strength, and yield strength. They all indicate the reliability and robustness of MEMS devices. These properties can be enhanced by optimal geometrical design.
The following types of materials are used for MEMS:
Due to the difficult conditions of micro-fabrication of MEMS materials, only a few materials meet the three main requirements for MEMS usage, namely good mechanical and electrical properties, adaptability to semiconductor fabrication technology, and properties that limit the development of stresses during micro-machining. [5]
Table 2. Mechanical properties of thin-film materials used for MEMS [5]
| Material | Elastic modulus (GPa) | Failure strength (GPa) | Thermal Conductivity (W/cm/oC) | Coefficient of thermal expansion (10-6/oC) | Specific heat capacity (J/kg/k) | Density (kg/m3) | Fracture toughness (Mpa.m1/2) |
| Diamond | 800 | 8.5 | 6.9 | 6.9 | 518 | 3500 | 5.9 |
| 3H-SiC | 400 | 7 | 3.5 | 3.3 | 1340 | 3200 | 3.8 |
| Si3N4 | 250 | 6.4 | 0.19 | 0.8 | 170 | 3100 | 1.8 |
| SiO2 | 70 | 1 | 0.001 | 0.55 | 937 | 2500 | 0.8 |
| SCSi (100) | 130 | 3.4 | 1.57 | 2.33 | 706 | 2300 | 1 |
| SCSi (110) | 168 | 7 | 1.57 | 2.33 | 706 | 2300 | 1 |
| Poly-Si | 159 | 1.65 | 0.34 | 2.8 | 706 | 2300 | 1.2 |
| Tungsten | 410 | 0.7 | 1.78 | 4.5 | 135 | 19300 | 44 |
| Aluminium | 70 | 0.17 | 2.36 | 25 | 899 | 2700 | 20 |
| Nickel | 185 | 0.4 | 0.899 | 13 | 444 | 8910 | 95 |
| Copper | 120 | 0.25 | 3.98 | 16.6 | 386 | 8960 | 85 |
| Titanium | 110 | 0.5 | 0.2 | 8.5 | 522 | 4510 | 70 |
| SU8 | 3 | 0.04 | 0.002 | 52 | NA | 1164 | NA |
| Polyimide | 8 | 0.04 | 0.001 | 20 | 1100 | 1420 | 3.9 |
| PVDF | 2.3 | 0.05 | 0.002 | 140 | 1500 | 1780 | 3.2 |
| PMMA | 2.4 | 0.08 | 0.002 | 80 | 1466 | 1200 | NA |
Due to the minute dimensions of MEMS, the fabrication method employed can greatly affect the properties of the MEMS device. Fabrication of MEMS falls into three main categories: bulk micromachining, surface micromachining and high-aspect-ratio micromachining [1].
This process involves the partial removal of the main (bulk) substrate. It uses wet unidirectional etching or dry etching methods to create holes, grooves, and channels in MEMS materials. Silicon and quartz are typically used for wet etching while silicon, metals, ceramic and plastics are used for dry etching.
This process involves the progressive addition of a material in the form of thin-film layers atop a base layer, usually a silicon wafer. These layers may be the structural material (typically polysilicon, silicon nitride, and aluminium) or a sacrificial material to be removed later. They are temporary fillers and when eventually removed, creates the desired empty spaces within the structure.
This process involves the replication of microstructures in metals from moulded parts through several methods that may include LIGA or laser micro-machining. Common materials amenable to this process are electro-formable metals or plastics, such as polyamide, acrylate, polycarbonate, and styrene.
Technologies arising from the application of MEMS have already proliferated great aspects of human life as can be seen from the table below.
Table 3. Applications of MEMS [1]
| Automotive | Electronics | Medical | Communications | Defence |
| Internal navigation sensors | Disk drive heads | Blood pressure sensor | Fibre-optic network components | Munitions guidance |
| Air conditioning compressor sensor | Inkjet printer heads | Muscle stimulators & drug delivery systems | RF Relays, switches and filters | Surveillance |
| Brake force sensors & suspension control accelerometers | Projection screen televisions | Implanted pressure sensors | Projection displays in portable communications devices and instrumentation | Arming systems |
| Fuel level and vapour pressure sensors | Earthquake sensors | Prosthetics | Voltage-controlled oscillators (VCOs) | Embedded sensors |
| Airbag sensors | Avionics pressure sensors | Miniature analytical instruments | Splitters and couplers | Data storage |
| "Intelligent" tyres | Mass data storage systems | Pacemakers | Tunable lasers | Aircraft control |
However, there is still room for growth, especially from the numerous challenges that prevent MEMS from attaining its full potential. Some of these stumbling blocks include [1]:
Despite the above challenges, MEMS technology has great potential as the benefits of its application are evident if the economic obstacles of price reduction and research can be overcome.
[2] http://scme-nm.org/files/History%20of%20MEMS_Presentation.pdf
[3] Walsh, S., Linton, J., Grace, R., Marshall, Knutti, S., MEMS and MOEMS Technology and Applications, edited by Rai Choudry, P., SPIE – The International Society for Optical Engineering, Bellingham, WA, Ch. 8, 2000.
[5] https://pdfs.semanticscholar.org/9188/d5ad43ba76d4b4f27956713d864996346b0f.pdf