June 29, 2011 — The electronics industry is moving toward miniaturization with very high delivery capacity/multi-tasking products. These products are continually demanding more in application capabilities within smaller spaces, with entire systems on one chip. Micro-electro-mechanical systems (MEMS) provide a means of developing "smart" products that the world has never before seen. The technology basically brings life to devices, allowing them to not only sense, but to control full systems from silicon-based microsensors and microactuators that are the size of a pinpoint.
As MEMS continue to revolutionize the world of electronic devices, replacing FR4-based sensor assemblies with nano-systems on a common silicon substrate, a new challenge arises: how to protect them. Standard conformal coatings that were sufficient for FR4 board assemblies are insufficient to protect the tiny, fragile parts of MEMS assemblies. Spray, dip, and brush-on conformal coatings are not able to meet the MEMS manufacturers’ needs.
Enter parylene
Parylene is an alternative to industry-standard conformal coating materials. With several formulations, including a new high-temperature, UV-stable variant, parylene offers lightweight, long-term protection. The latest developments in parylene have produced new formulations that offer even smaller molecular structures and superior thermal, UV and dielectric protection. While parylene has been in use for decades on all basic assemblies, particularly those in the military and aerospace areas, new adhesion-enhancing pre-treatments have been developed to meet the adhesion challenges of coating some of the unique materials used today in the latest electronic and medical sub-assemblies. Parylene coatings are at times the only method to effectively protect micro-assemblies and enhance life performance.
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| Figure 1. The basic conformal nature of parylene compared to other types of conformal coatings. SOURCE: Specialty Coating Systems. |
The major benefit of parylene is its ultra-thin nature, resulting from a vapor deposition process. The coating ensures superior barrier properties, but in micron-level thickness that coats into even the smallest profiles and crevices of a MEMS device for complete protection (Fig. 1).
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| Figure 2. The parylene vapor deposition process ensures barrier properties in micron-level thickness that coats into even the smallest profiles and crevices of a MEMS device for complete protection. SOURCE: Specialty Coating Systems |
Because there is no liquid phase in the deposition process, there are no subsequent meniscus, pooling, or bridging effects as seen in the application of liquid coatings, thus dielectric properties are never compromised. The molecular build-up of parylene coatings also ensures a uniform and conformal coating at the thickness specified by the manufacturer (Fig. 2). Because parylene is formed from a gas, it also penetrates into every crevice, regardless of how seemingly inaccessible. This ensures complete encapsulation of the substrate without blocking small openings.
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| Figure 3. Magnified view of a MEMS device coated with parylene. The teeth are 2µm wide and spaces between each measure 4µm. SOURCE: Image courtesy of Sandia National Laboratories, SUMIT Technologies, www.mems.sandia.gov. |
Parylene can be formulated to suit many needs. A few key benefits of parylenes include:Moisture protection: parylene protects components from damage caused by exposure to humidity, moisture, chemical and fluids; High-temperature resistance: parylene can consistently withstand 350°C operating temperature, short-term exposures to 450°C; Small molecular structure: the extremely small molecular structure allows parylene to ingress deeper through open areas on the top or bottom of any package regardless of the size or complexity of integrated devices (Fig. 3); Low dielectric constant and dissipation factor: parylene has an extremely low dielectric constant and dissipation factor, enabling it to provide small, tight packages with dielectric insulation via a thin coating.
MEMS in harsh environments
Thermal protection. MEMS that require thermal stability in extreme environments are highlighted in applications such as hydrocarbon drilling sensors. Used in oil, gas and other types of hostile drilling and deep-penetration exploration, all components on the assembly need to be resistant to extreme temperatures, pressures and fluids exposure. Parylene coating provides this protection for minute MEMS sensors without changing the dimension of the device or affecting the ability of the sensor to perform in any way.
Thermo-electric devices also benefit from parylene because of their use in applications that vary between hot or cold environments, interchangeably. The device can operate efficiently without a thick coating of material. This is also true for MEMS used in power switches for aerospace and space applications. Parylene protects from extreme altitude and deep space environments, and is already a mil-spec approved coating.
Moisture protection. When MEMS are used in devices where electrical and/or moisture damage can occur, parylene is a way to not only protect the device assembly, but extend life in these environments. One application is for protecting acoustic, sound delivery and sound enhancing devices where moisture can degrade and impede the performance of the device. Parylene can offer an additional level of protection during the reflow assembly steps of the devices.
The increasing use and development of microfluidic devices, or “lab on a chip,” where fluids are analyzed, dispensed or moved in micron thick channels, requires moisture protection of the channel materials. Parylene can provide a uniform, conformal and pinhole-free coating for the protection of the channel and the entire device. When the microfluidic devices are designed as an implant, parylenes are biocompatible, biostable, and well recognized in the medical industry.
Another application that is rapidly coming into its own is in the area of energy harvesting devices (solar, other PV, wind, etc.) where moisture can reduce the energy pickup effectiveness of the device.
Emerging applications. Based on MEMS wafers, the growing development of “MOEMS” (micro-opto-electromchanical system is also ideal for parylene because, when using parylene for added protection, there are no fillers to interfere or reduce the optical signals or transmission rate between devices.
When MEMS wafers are conformally coated with parylene, the parylene can be etched away using proven etching technology in the fabrication of the devices. Because parylene is applied in the micron or sub-micron range, this allows it to be used where other conformal coating material is not considered. Parylene can reduce the need for additional packaging for protection of the device.
Conclusion
Whether pressure, fluid, temperature, motion, load, torque or others, all types of micro-sensors need protection to work effectively. When dealing at the micro-level, all formulations of parylene products provide this level of protection. All manufacturers moving from basic FR4 technology to MEMS sensor devices in their assemblies should consider parylene as a protective alternative that can provide higher reliability and longer life to their end products, and thereby reduce overall cost of maintenance, repair and replacement for end customers.
Acknowledgment
SUMIT is a trademark of Sumit Technologies.
Alan Hardy received his BA from Bethany College in Lindsborg, Kansas and is the Automotive, Electronics & Military Market Manager at Specialty Coating Systems, 7645 Woodland Drive, Indianapolis, IN 46275 USA; ph.: 317-244-1200; email [email protected]; http://www.scscoatings.com