Low-Cost MEMS Packaging Using Sacrificial Polymer-Based In-Situ Airgap Creation

The improvements in microelectromechanical systems (MEMS) have been traditionally focused more on the functionality and sensitivity of devices. However, packaging of MEMS devices has become a bottleneck mainly due to a wide range of device dimensions and packages being device-specific. Even though MEMS packaging sector is estimated to grow 2x faster than integrated circuit (IC) packaging sector, MEMS packaging is still one of the least explored components of MEMS technology compromising up to 20-40% of the total material and assembly cost [1, 2, 3]. One essential concern is the cleanliness of surface of MEMS devices after packaging, since the contaminants might interfere with the device performance and reliability. In order to address these issues, we pursue sacrificial polymer-based airgap MEMS packaging as a low-cost, clean, IC-compatible, high volume, semi-hermetic, wafer-level packaging solution. 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 airgap creation process uses complete thermal decomposition of a sacrificial polymer, poly(propylene carbonate) (PPC), encapsulated with overcoat materials, such as BCB (Cyclotene) and/or epoxy-molding compound (EMC), and permeation out of PPC decomposition products through the overcoat materials. To this end, we have previously demonstrated a minimal decrease in resonant frequency (<0.1%) and a small change in the quality factor (<5.2%) in airgap packaged MEMS resonators using PPC with BCB overcoat (i.e. no EMC overmolding) [4]. In this study, in order to minimize the material and processing cost and to simplify the previously developed packaging process, we will use a thin layer of patterned BCB only for patterning non-photosensitive PPC with O₂ plasma, not for the overcoat. MEMS resonators with patterned PPC (and BCB) will be wire-bonded on a lead-frame package, and finally EMC overmolding will be done during which the decomposition of PPC and the curing of EMC and BCB will be achieved simultaneously, i.e. “in-situ airgap creation”. Apart from that, PPC will be subjected to a purification procedure by which lower decomposition temperatures (T_d) were recently obtained in our group compared to unpurified PPC (T_d difference about 20°C).  After obtaining the complete airgap packaged MEMS resonators, an aging study will be performed in order to determine the change in resonant spectra over time.

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. E. Uzunlar, Paul A. Kohl, “Low-cost MEMS Packaging using Polymer-based Air-gaps”, 225^th Electrochemical Society (ECS) Meeting, Orlando, FL, May 11^th – May 15^th 2014.