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Microelectromechanical systems (MEMS) are devices combining very small mechanisms (up to 1mm/0.04”) with electronic integrated circuits.
MEMS are assembled from mechanical components with the sizes 1-100μm (0.00004-0.004”).
The MEMS devices:
The most successful MEMS devices, which have been widely used are: accelerometers for air bag systems (Analog Devices, Inc.) and Digital Mirror Devices (Texas Instruments, Inc.).
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MEMS are fabricated from polycrystalline silicon (polysilicon).
The advantages of polycrystalline silicon:
The main disadvantage of polycrystalline silicon is the high surface energy of the oxide film (SiO2) forming on the silicon surface. Due to the high surface energy silicon is prone to a strong adhesion to the counterparts, which results in a higher friction and an increased wear.
The problem of high surface energy is aggravated by the high surface-to-volume ration characteristic for the microscale systems. As compared to conventional mechanical devices the components of MEMS are 1000 times smaller therefore the surface-to-volume ratio of MEMS is 1000 times larger. As a result the forces proportional to the surface area (capillary, viscosity, electrostatic, van der Waals) become 1000 times grater in relation to the forces proportional to the volume (inertia, gravity, electromagnetic).
High surface energy of polysilicon makes it hydrophillic. This property is extremely undesirable for MEMS. Water used in the fabrication processes or water from a humid atmosphere penetrates between the MEMS components driven by the capillary force. The capillary force sharply invcreases the adhesion between the surfaces and may prevent the device from normal operation.
However even if the device is dry high surface energy of the polysilicon components results in a significant permanent adhesive attraction (stiction) due to an action of other surface forces (van der Waals, electrostatic) developed between the closely adjacent surfaces of the miniature mechanism.
Stiction is a unintentional permanent adhesion which may cause failure of microelectromechanical systems.
The effect of stiction may be considerably reduced by a deposition of a coating decreasing the surface energy of polycrystalline silicon.
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Attraction of adjacent MEMS surfaces may be reduced by roughening them (a physical method). Roughened surfaces have lower effective area of contact.
One of the most effective methods of chemical modification of the surface properties of polysilicon is applying self-assembled monolayers (SAM).
A self-assembled monolayer (SAM) is a thin film built of organic molecules having two different ends: a hydrophilic head group and a hydrophobic functional group. The head group tails bond to the substrate surface whereas the functional tails form a modified hydrophobic surface with low surface energy and therefore reduced effect of stiction (by three to four orders of magnitude).
Two organic substances have been proposed for formation of self-assembled monolayers on MEMS: octadecyltrichlorsilane (ODTS) and perfluorodecyltrichlorosilane (FDTS).
SAS may form on the substrate surface either from nonaqueous solutions or by Chemical Vapor Deposition (CVD).
Air bag inertia sensors of Analog Device are coated by using evaporation of phenylsiloxane which resists buildup of electical charge and survives elevated packaging temperatures.
ODTS films have a thickness ~2.8 nm.
The thickness of FDTS films is ~1.4 nm.
Self-assembled monolayers are thermally stable in oxygen containing atmosphere up to 752°F (400°C).
Another type of anti-stiction coatings is carbon based.
Surmet Corporation have proposed a low stress ultra hard amorphous Diamond-Like Carbon (DLC) coating providing ultra low coefficient of friction (0.05) and reduced wear. The method of conformal-plasma induced chemical vapor deposition (CPI-CVD) is used for the coatings deposition.
A potential alternative of carbon based low friction coating is ultrananocrystalline diamond (UNCD) - a coating build of very small (3-5 nm) diamond crystalls.
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