Biomaterials for MEMS

Introduction 3

on the catheter surfaces, is said to create a favorable environment for bacteria

to grow. While the catheters should provide required strength to maintain their

mechanical structures integrity, the prevention of surface fouling is also critical for

device design.

1.2 MICROMACHINING OF BIOMATERIALS

MEMS is a technology that uses micromachining techniques to form microscale

devices, such as accelerometers,

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optical lens,

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RF (radio frequency) devices

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and microfluidic devices. Micromachining uses thin-film deposition and etching

techniques developed by the IC (integrated circuit) industry to fabricate small

mechanical parts monolithically on a substrate. Limitations, such as processing

temperature and chemicals used in established IC fabrication processes, restrict

materials to silicon, silicon dioxide, silicon nitride, aluminum and refractory

metals. Conventional micromachining methods start with a bulk material, for

example, a silicon wafer, following by thin-film deposition, photolithography and

etching. The process can be repeated, with some materials forming sacrificial

layers and structures, to form more complex 2-D or 3-D structures. The level of

complexity of a micromachining process is often determined by the number of

photolithography masks used in the processes.

Historically, micromachining process can be categorized into two groups: (1)

bulk micromachining and (2) surface micromachining. Bulk micromachining is often

involved in etching of a silicon wafer itself and use the single crystalline planes

in silicon to form mechanical structures. Examples include early ink-jet printing

nozzles, inertial sensors, as well as microfluidic valves and pumps. On the other

hand, surface micromachining uses silicon substrates only as a mechanical sup-

port, the add-on thin-films, such as polycrystalline silicon (poly-si), silicon nitride

and metals are the actual mechanical components. Examples include scanning

optical mirrors, inertial sensors and radio frequency switches. Typically, bulk

micromachining consumes more silicon real estate than surface micromachining,

but may require less processing steps. Bulk-micromachined structures are gen-

erally simpler and provide generic advantages using single crystalline structures

such as smoother planes, higher Young’s Modulus and reliability. Surface micro-

machining, on the other hand, offers potentials for complex structures to satisfy

nonconventional functionality and requirements, possibility to be integrated with

complementary metal oxide semiconductor (CMOS) circuitry and ability to adapt

to exotic materials.

Micromachining of biomaterials initially used bulk-micromachined silicon

molds.

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Silicon wafers were etched into micro-scale structures as molds as

they offer precision in the molds by which the conventional metal drilling or

quick prototyping methods may not offer. Casting, hot-embossing and micro-

molding have been used

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to mold biomaterial (mainly polymers) to desired

shapes. Hot-embossing and micro-molding are rather mature technologies and

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