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MicroCap, WLCSP for RF Filters
Keywords: MicroCap, WLCSP , RF filters
Explosive growth of Wireless communication devices has triggered a need for high performance ever smaller RF filters. The biggest wireless market (Cellular phones) has seen dramatic reduction in RF filter size and a move towards the ultimate solution: WLCSP. This paper will describe the development and evolution of WLCSP in Avago technologies (internally known as MicroCap). The technology uses wafer bonding of a lid (Cap) Si wafer to a filtering (FBAR) Si wafer. After review of multiple bond options like Solder gasket and organic bonding Avago has selected unconventional cold Au-Au bond along with (very small size) raised Silicon (Mesa) gasket. RF signals are routed using plated via-holes while RF performance is maintained; the impact on RF performance is negligible as it is below detection level. Hermeticity, the hardest parameter to achieve, has been maintained to high cell phone requirements. The technology has been proven in (very) high volume Cell-phone market environment in the last five years. At later years the technology has also been applied to GaAs wafers. The paper will discuss MicroCap development, why the market was not ready for it; hence the use of package within another package and evolution from inception to today's high volume Cell phone business. More than 1 billion MicroCap filters have been sold.
Moshe Gat,
Avago Technologies
San Jose, CA

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