Low-Cost MEMS Packaging Using Polymer-Based Air-Gaps
Authors: Uzunlar Erdal , Kohl Paul A 
Published: February 2020

Packaging of microelectromechanical systems (MEMS) is essential for preserving device performance, providing physical protection and enabling electrical connections to devices. MEMS packaging sector is estimated to grow 2x faster than the integrated circuit (IC) packaging sector, because of ever-increasing penetration of MEMS technology into mobile and consumer applications in recent years [1]. Despite its growth, MEMS packaging is still one of the least explored components of MEMS technology [2]. Moreover, MEMS packaging can compromise up to 20-40% of the total material and assembly cost [3]. Besides, conventional MEMS packaging techniques involving ceramic, metal or plastic packages has challenges either of size limitations, high-temperature requirements, and packages being device-specific [4]. In addition, the surface of the MEMS devices should stay clean after packaging because the atmospheric contaminants and moisture can create dampening effects and oxidation, which might interfere with device performance and reliability. Therefore, there is a clear need for low-cost, clean, IC-compatible, high volume, wafer-level packaging solutions.
The goal of this study is to develop a MEMS packaging technique using polymer-based air-gaps as a cost-effective method. The intention is to stabilize and secure the movable MEMS components inside a protective envelope so that the MEMS chip can then be treated like an integrated circuit (IC) and enjoy the benefits of low-cost, high-volume IC packaging, such as lead-frame, or plastic overmolding packaging. The air-gap creation process uses thermal decomposition of patterned sacrificial polymer layer, poly(propylene carbonate) (PPC), which is encapsulated with an overcoat material, BCB (Cyclotene), through which PPC decomposition products permeate out, finally leaving an air cavity on top of MEMS device enclosed with overcoat. The process involves coating MEMS device with PPC, coating and patterning a thin pattern-transfer layer of BCB overcoat on top of PPC, reactive-ion etching by O2 plasma for patterning PPC, coating and patterning overcoat layer of BCB, then simultaneous thermal decomposition of PPC and curing of BCB (at around 250°C for 2-4 hours in N2 purged oven). PPC acts a temporary place-holder for defining air-gap region, whereas BCB provides mechanical stability to package. The application of the process on bare silicon substrates enabled 5-10 μm high air-gap creation in square or circular features up to 2 mm in size with no cracking or deformation in BCB overcoat. BCB is found to be chemically compatible with PPC providing uniform, conformal and adherent coating on top of PPC. Nanoindentation measurements revealed a mechanically stable and flexible BCB overcoat layer on top of the air-gap. The surface of silicon in the air-gap region was found to be clean after removal of BCB overcoat, with only a 5-10 nm thick high molecular weight hydrocarbon by X-ray photoelectron spectroscopic (XPS) measurements. The packaging process was also performed on actual working MEMS resonators of sizes 200 μm x 100-160 μm. Comparing the before-packaging and after-packaging resonant spectra measurements, we have observed a minimal decrease in resonant frequency (<0.1%), and little-to-no change in quality factor (<5.2%) in working MEMS resonators. Current focus is on implementing the developed packaging process into a lead frame packaging scheme with epoxy molding to achieve near-hermetic, actual MEMS devices. During epoxy molding, in situ decomposition of PPC and curing of BCB will occur simultaneously with curing of epoxy molding compound.
Overall, a new MEMS packaging technique has been studied in order to achieve low-cost, clean, IC-compatible MEMS packages. The technique is easy-to-implement, and readily scalable to wafer-level packaging. The packaging of actual MEMS devices with the use of the developed packaging technique along with conventional lead frame packaging with epoxy molding is underway.
References
1. MEMS Packaging Market and Technology Trends Report, Yole Développement, 2012.
2. M. Gaitan, MEMS Technology Roadmapping, Nano-Tec Workshop 3, 31 May 2012.
3. R.H. Grace, M. A. Maher, “MEMS: Think outside the chip... at the package level”, Hearst Electronic Products 2010, http://www.electronicproducts.com/Packaging_and_Hardware/Prototyping_Tools_Equipment_Services/MEMS_Think_outside_the_chip_at_the_package_level.aspx
4. P. Monajemi et al., “Characterization of a Polymer-Based MEMS Packaging Technique”, 11th International Symposium on Advanced Packaging Materials: Processes, Properties and Interface, 2006.