Polymers are long chains of carbon (or sometimes silicon) atoms with variouschemical side groups attached to the carbon .
If the chains are not crosslinkedby covalent bonds, they are able to move relative to each other at elevated temperature under applied stress.
Such materials reharden upon cooling and are called thermoplastics.
The temperature above which flow readily occurs is the glass transition temperature, which varies with the length of the molecules and the type of side groups.
PMMA [poly(methyl methacrylate)], polypropylene, polyvinyl chloride, acrylic, and other thermoplastics are used in sheet form as a substrate for micromachining.
Heating above the glass transition temperature enables molding or embossing under pressure from a master for some of these materials.
Layers of polycarbonate and acrylic, with channels already formed in their surfaces by hot embossing or conventional machining, have been thermally bonded together for microfluidic systems.
In MEMS, thick layers of PMMA have also been spin-coated and used as a photoresist.
Polymer substrates have not been used as much as silicon in micromachining, but have some advantages, perhaps the most important being lower cost.
The processing temperatures allowed are much lower than for silicon and many glasses, but suitable fabrication processes have been designed, particularly for biological applications.
Polymers are in general less stiff than inorganic materials.
Polyimide is a material that is most often used in the form of sheets 7 to 125 μm thick, but can also be spin-coated in films a few micrometers thick. Polyimide is relatively inert, is a good electrical insulator, and can be exposedto a wide range of temperatures, roughly –250º to +400ºC, for at least a short time.
In the electronics industry, polyimide has been used as a flexible substrate for printed circuit boards and for hard disk drives.
In micromachining, sheets have been laser cut to form microfluidic devices, while spin-on films have been used as resists, sacrificial layers, and a wafer-bonding adhesive.
Other polymers finding application in MEMS include parylenes and silicones.
Parylenes are deposited by chemical-vapor deposition to form a conformal coating.
There are several forms of parylene due to variations in the chemical structure . Like polyimide, parylenes are fairly inert chemically and form a barrier to the flow of water and other vapors.
Silicones are different from most other polymers in that the backbone chain of atoms is silicon rather than carbon.
Silicones are very compliant and have been used as the deformable membrane in valves , as well as being a common die-attach material in packaging.