Materials Used in MEMS

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.

In this article, you will learn about:

  • The history of MEMS
  • MEMS materials and relevant properties
  • MEMS fabrication methods
  • Future materials and application

The history of MEMS

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

MEMS materials and relevant properties

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

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.

Inelastic properties

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.

Strength properties

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:

  • Non-metals: Silicon, germanium, and GaAs
  • Metals: Nickel and aluminium
  • Polymers: SU8 and polyamide
  • Ceramics: Diamond, SiC, SiO2, Si3N4.

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

MEMS fabrication methods

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].

Bulk micromachining

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.

Surface micromachining

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.

High-aspect-ratio micromachining

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.

The future of MEMS technology

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]:

  • Absence of standardisation in testing methods and techniques of fabrication
  • The difficulty of separating machining techniques from the design of MEMS devices
  • Accessibility to MEMS fabrication facilities is still very scarce, which hinders research and innovation in the field

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.





[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.