The NXP-TSMC Research Center introduced an innovative method for packaging MEMS components during the "MEMS 2008" conference in the United States on January 17th. This groundbreaking technique, presented under paper number 156-Th, utilizes materials commonly found in the wiring layers of advanced CMOS LSIs. The approach aims to bridge the gap between MEMS packaging and traditional CMOS manufacturing, making it compatible with TSMC's production lines.
This method leverages low-k materials—typically used in high-performance LSI interconnects—for the encapsulation of movable MEMS structures within a cavity. By integrating MEMS packaging into the existing CMOS process, the technology simplifies fabrication and enhances compatibility with mainstream semiconductor manufacturing.
In advanced LSIs, low-k materials are employed to reduce dielectric constants, minimizing crosstalk and enabling faster signal transmission. However, when applied to MEMS packaging, the focus shifts to their porous characteristics. These pores play a critical role in the sacrificial layer etching process.
Traditionally, MEMS packaging involves creating a cavity around the device and covering it with a lid. A sacrificial layer is then etched away through a small opening in the cover. By using a porous material as the lid, the sacrificial layer can be removed via vapor-phase etching, which offers better control and precision. Afterward, a non-porous film is deposited on top to ensure a hermetically sealed package.
During this process, hydrofluoric acid (HF) is used for vapor-phase etching. The etching rate for the porous material is extremely slow—only 0.14 nm per minute—which allows for excellent selectivity over the sacrificial silicon dioxide layer. Even with a porosity as low as 7%, the sealing material remains effective, preventing unwanted infiltration into the cavity.
The low-k material used in this project is “Black Diamond†from Applied Materials, known for its superior performance in advanced ICs. The company plans to first implement this packaging technology in silicon oscillators, which have already been developed for RF applications. These devices were previously showcased at the IEDM 2007 conference in December 2007, marking a significant step forward in the integration of MEMS and CMOS technologies.
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