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Near Hermetic Wafer Level Packaging using LCP Adhesive Layer
Keywords: Wafer Level Packaging, Liquid Crystal Polymer, Near Hermetic
One issue when using RF MEMS in todays electronics is the requirement of a low loss, low cost hermetic package. The packaging methodology discussed herein pertains to wafer level packaging of RF circuits using capacitive RF MEMS switches fabricated on alumina. In order to maintain switch lifetime, the internal package humidity level must remain at less than 1% RH at atmospheric pressure. To accomplish this, a packaging methodology was developed using liquid crystal polymer (LCP) as an adhesive between two non-planar substrates. In this work a glass lid wafer was bonded to an RF MEMS alumina wafer with 280 fabricated die. The bond surface is highly non planar as each package has 10 I/O transitions through the bond ring. The LCP bond ring is 10 mils wide and constructed on the glass wafer out of 2 or 4 mil thick LCP laminate. An added benefit of LCP is the ability to control the squeeze-out of the bonding material. Unlike solders, which reflow into the cavity and are difficult to control, LCP flow can be controlled using bonding pressure and temperature. The LCP is bonded either directly to the alumina or to a metal thin film bond ring on the alumina. In this paper a discussion of the bonding methodology, method of evaluating the quality of the package, and results from initial and environmental testing of the package of a statistically substantial sample will be discussed. The results show that while the package is not truly hermetic, the method eliminates all initial moisture and prevents diffusion of moisture into the package under accelerated lifetime test which included high humidity environments. Also discussed is the RF performance of the package. This packaging method provides an acceptable low loss, low cost wafer level approach for near hermetic packaging of RF MEMS devices.
Cody Moody, Sr. Electrical Engineer
Dallas, TX

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