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Wafer-Level Packaging Method for RF MEMS Applications Using Pre-Patterned BCB Polymer

Institute of Microelectronics, Tsinghua University, Beijing 100084, China
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Micromachines 2018, 9(3), 93; https://doi.org/10.3390/mi9030093
Received: 21 January 2018 / Revised: 19 February 2018 / Accepted: 21 February 2018 / Published: 25 February 2018
(This article belongs to the Special Issue Wafer Level Packaging of MEMS)
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Abstract

A radio-frequency micro-electro-mechanical system (RF MEMS) wafer-level packaging (WLP) method using pre-patterned benzo-cyclo-butene (BCB) polymers with a high-resistivity silicon cap is proposed to achieve high bonding quality and excellent RF performance. In this process, the BCB polymer was pre-defined to form the sealing ring and bonding layer by the spin-coating and patterning of photosensitive BCB before the cavity formation. During anisotropic wet etching of the silicon wafer to generate the housing cavity, the BCB sealing ring was protected by a sputtered Cr/Au (chromium/gold) layer. The average measured thickness of the BCB layer was 5.9 μm. In contrast to the conventional methods of spin-coating BCB after fabricating cavities, the pre-patterned BCB method presented BCB bonding layers with better quality on severe topography surfaces in terms of increased uniformity of thickness and better surface flatness. The observation of the bonded layer showed that no void or gap formed on the protruding coplanar waveguide (CPW) lines. A shear strength test was experimentally implemented as a function of the BCB widths in the range of 100–400 μm. The average shear strength of the packaged device was higher than 21.58 MPa. A RF MEMS switch was successfully packaged using this process with a negligible impact on the microwave characteristics and a significant improvement in the lifetime from below 10 million to over 1 billion. The measured insertion loss of the packaged RF MEMS switch was 0.779 dB and the insertion loss deterioration caused by the package structure was less than 0.2 dB at 30 GHz. View Full-Text
Keywords: wafer-level packaging; adhesive bonding; photosensitive BCB; RF MEMS wafer-level packaging; adhesive bonding; photosensitive BCB; RF MEMS
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Gong, Z.; Zhang, Y.; Guo, X.; Liu, Z. Wafer-Level Packaging Method for RF MEMS Applications Using Pre-Patterned BCB Polymer. Micromachines 2018, 9, 93.

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