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.
Highlights
Radio-frequency micro-electro-mechanical system (RF MEMS) devices, such as switches, tunable capacitors and resonators, have movable and fragile parts that must be protected from damage and contamination during wafer handling and dicing [1,2,3]
The MEMS metal-contact switches are sensitive to moisture and particles from ambient environment, as the contamination of the contacts has a detrimental effect on the switching reliability
In order to obtain a BCB layer with uniform thickness and a flat surface, which plays an important role in achieving a robust package with excellent bonding quality, an improved packaging process is proposed. This process is based on pre-patterned BCB, which means that the BCB polymer is patterned before the etching of the housing cavity in the cap wafer to form the sealing ring and the bonding layer
Summary
Radio-frequency micro-electro-mechanical system (RF MEMS) devices, such as switches, tunable capacitors and resonators, have movable and fragile parts that must be protected from damage and contamination during wafer handling and dicing [1,2,3]. The package must offer encapsulation with a low gas leak rate in an inert ambient gas, such as nitrogen, at atmospheric pressure to ensure the long-term stability and reliability of the MEMS switches. To solve these problems, the packaging is preferably implemented on a wafer level prior to die singulation, which is known as the wafer-level package method [6,7]. A new approach for the wafer-level bonding based on a pre-patterned BCB process is proposed to offer a fine bonding interface on a severe topography surface for excellent package quality.
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