Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 3M

• Polymers are long chains of carbon (or sometimes silicon) atoms with variouschemical side groups attached to the carbon [13].
• 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[14].
• 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 [15]. 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 [15], as well as being a common die-attach material in packaging.