By Amphenol All Sensors Corporation
Microelectromechanical systems (MEMS) pressure sensor descriptions often include a statement such as “intended for use with non-corrosive, non-ionic working fluids such as air or dry gases.” Following this guideline, users can expect a long working life for a MEMS pressure sensor. But what if the application does not fall within the guidelines? This white paper will explain why protection is required for MEMS pressure sensors and discuss potential solutions, including Parylene.
Why MEMS Pressure Sensors Require Additional Protection
Introduced over 40 years ago, MEMS pressure sensors using piezoresistive technology (PRT) provide a solution to more pressure sensing needs than any other technology. The reasons are simple: PRT allows absolute, gage and differential pressure measurements that address both high and low volume application. Additionally, these sensors are highly accurate, highly repeatable, and meet the cost objectives better than other alternatives.
Many MEMS pressure sensors are available in plastic packages, but some are metal or ceramic. Wire bonds connect the electrical contacts on the pressure sensor’s die surface to the leads of the package to interface to external components.
In silicon pressure sensors, a passivation layer applied near the end of the wafer manufacturing process protects implanted or diffused elements on the top surface but the diaphragm area is masked to avoid the dissimilar material interface and dampening effect (with lower response time) that would be caused by the protective glass layer. Thus, subsequent protection at the package level is frequently required, especially for harsh applications. An example of the cross-section with a protective gel is shown in Figure 1.
Figure 1. Cross-section and typical materials of plastic-packaged MEMS pressure sensors.
Potential Protection Techniques
Different solutions have been developed to protect pressure sensors from harsh media, including oil filled, hermetically-sealed stainless steel, O-rings/diaphragm, hydrostatic methyl-silicone gels, and more. While the sensitive sensor surface must be isolated from harmful media, the pressure still must be accurately transmitted to the sensor’s diaphragm for static and dynamic measurements. Air voids in oil or gel fills can cause inaccurate measurements. As long as the sensor manufacturer’s recommendations are followed, silicone gel coatings can typically be used down to pressures as low as 5 psi, but for many applications in lower ranges, such as ±30inH2O (±1.08psi) to ±1inH2O (±0.036psi), a different approach is required.
An alternative harsh media protection solution is Parylene. This thin-film coating provides a moisture, chemical, and dielectric barrier and offers thermal and ultra-violet (UV) stability and dry-film lubricity. Parylene is the thinnest effective coating technique available. One of Parylene’s important characteristics is its ability to coat all surfaces, including penetration deep into multi-layers and crevices. Unlike the added weight of cavity-filling liquids, Parylene coating has minimal impact on the sensor’s accuracy and repeatability.
Figure 2. Cross-section and typical materials of a plastic-packaged MEMS pressure sensor with Parylene coating shown in red.
Parylene as an Optimal Cost-Benefit Protection Solution
Parylene is applied using a specialized chemical vapor deposition (CVD) process to provide a highly uniform, conformal coating.
In the vapor deposition polymerization process, products to be coated are placed in the product chamber of the specialized vacuum deposition equipment.
Next, dimer, the raw material, is placed upstream into the system. Dimer (an oligomer, or molecular unit with repeating units consisting of two monomers) is a solid, granular material. The dimer is heated under vacuum until it is vaporized into a dimeric gas to pyrolyze (or thermally decompose) it into a monomeric gas.
Finally, the monomeric gas flows into the ambient temperature chamber and polymerizes on all surfaces in a thin, transparent film. At this point, the process is complete, since no post-deposition curing is required.
Because it is applied as a gas, Parylene coats all exposed substrate surfaces and penetrates small crevices and tight areas on multi-layer components, providing complete and uniform encapsulation. The target thickness of the polymer coating is determined by the application and the coating properties desired. Typical thickness for the chemically-inert material is in the microns range.
Getting Parylene Into/Onto Production Pressure Sensors
Unlike other pressure sensor suppliers offering a Parylene coating, All Sensors performs this process in-house.
The Parylene that All Sensors uses exhibits faster deposition rates than other types of Parylene. With its in-house process, All Sensors successfully coats pressure sensors as low as ±1inH2O (±0.036psi) full scale (FS) with sensitivities in the range of 4.5mV/V/inH2O.
Examples of Sensors with Parylene Protection in Harsh Environments
In addition to medical applications, Parylene is ideal for industrial applications such as flow controllers, gas/propane appliances, and more.
Medical Applications
Since Parylene is chemically inert, it is biocompatible and safe to use as a coating for medical equipment. Also, it has minimal potential for infection or distorting data readings. Parylene retains its compositional stability and performance in the presence of bodily fluids and tissues.
Industrial Applications
Monitoring or controlling methane, the primary constituent of natural gas, are natural applications for Parylene protected pressure sensors. For flow control, a sensor with a range as low as ±1inH2O (±0.036psi) may be required. As the cleanest burning fossil fuel, methane has great potential as an alternative to other fossil fuels. Controlling and monitoring methane, especially to detect leakage is essential for its safe usage.
Summary
Successfully surviving a wide range of applications requires addressing all of the application’s key parameters. For pressure sensors, this include the proper protection for harsh and sometimes corrosive media. For over 40 years, Parylene has been used to provide this protection to pressure sensors and other electronic products.
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