Parylene is proving to be an ideal material for sealing, insulating, and protecting electronic modules, devices, and circuit boards. It’s easy to apply, goes down in uniform layers, and stands up to many environmental hazards. These same properties also make Parylene an excellent choice for manufacturing micro-electromechanical systems (MEMS) – sensors, actuators, and structures forged from silicon using industry standard semiconductor-processing techniques. In fact, Parylene is helping MEMS designers overcome some of their toughest challenges, clearing the way for new features and functions and a fresh growth spurt for MEMS applications.
From the time silicon was first used to form mechanical structures and devices, designers have struggled with space constraints on both ends of the dimensional spectrum. MEMS devices – accelerometers, gyroscopes, and flow sensors, for example – typically contain many complex moving parts as well as signal processing and interface circuits, and all these elements must fit comfortably in packages scaled to size of tiny silicon chips. At the same time, every nook and cranny in the labyrinth of silicon micro-machinery must be accessible to fluids and light fields employed in the many processing steps. Parylene excels under such circumstances because it’s applied in a gaseous or vapor state under vacuum. The vapor deposition process uniformly coats even the most inaccessible surfaces, penetrating spaces as narrow as 0.01 mm. It also covers sharp edges, points, and exposed internal surfaces, resulting in a thin conformal layer that’s free of pinholes and impermeable to anything larger than 1.4 nanometers (nm).
Another challenge MEMS designers face stems from thermal and mechanical stresses imposed by the presence of sealing materials as well as the processes by which they’re typically applied. Thick heavy coatings, for example, can reduce the sensitivity and dynamic range of motion sensors and actuators. The effects of high processing temperatures, on the other hand, are potentially worse and may even be catastrophic. Parylene circumvents both problems. For one, it’s applied at room temperature and it doesn’t require curing like many other coating materials. It also minimizes mechanical stresses and loads because it can be applied at a precisely controlled thickness. Parylene layers form on substrates literally one molecule at a time, resulting in uniform films that can be anywhere from a few angstroms to several microns thick.
Even at a thickness of just 0.5 microns (µm), Parylene achieves a near impenetrable defense for the surfaces and structures it protects. Not only does it provide a pinhole-free moisture and chemical barrier, but also a biological barrier. What’s more, components sealed with Parylene are unaffected by solvents, including gasoline and acetone, and can easily pass a 100hr salt-spray test. Besides being chemically and biologically inert, Parylene offers outstanding wear and dry-film lubricity properties with a static coefficient of friction near that of Teflon – as low as 0.25 to 0.30. It’s also stable over a wide temperature range (-200‘C to +200‘C) and is extremely rugged, having high tensile and yield strength in the range of 50 to 70 MPa.
Parylene’s electrical and optical properties are also well suited for MEMS applications. Parylene is a good electrical insulator with high dielectric strength and high bulk and surface resistance. It also has negligible capacitive effect thanks to its low dielectric constant. This mitigates parasitic losses that would otherwise occur at high frequencies. As for its optical properties, Parylene is relatively transparent and can be used to coat LEDs, light sensors, mirrors, and lenses. It also withstands UV radiation and protects optical components from UV-induced damage. Parylene can be applied to most vacuum-stable materials, including optical plastics, metals, quartz, and semiconductors.
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