Low-Cost and Reliable Package for RF-MEMS Tunable Capacitor

The use of a MEMS variable capacitor makes it possible to reduce the electrical parts and system size of radiofrequency applications such as a multimode mobile system.

Toshiba has developed a package for an electrostatically actuated MEMS variable capacitor. Since the MEMS capacitor is required to be operated at atmospheric pressure in a dry atmosphere, we developed a wafer-level package (WLP) encapsulation structure with four thinfilm layers fabricated by conventional back-end-of-line (BEOL) technologies.

Furthermore, since the MEMS chip is actuated by a control IC chip that provides high voltage, we developed a stacked multichip package (MCP) to integrate the MEMS chip and the control IC chip. The process of the stacked MCP had to be optimized to handle the fragile WLP encapsulation. The newly developed stacked MCP is 4.5 mm × 4.5 mm in size and 0.8 mm in thickness.

The stacked MCP has sufficient reliability, and we confirmed that the MEMS variable capacitor operates normally even in a humid atmosphere.

Cross-sectional view of WLP encapsulation

Schematic view of stacked MCP, Ball-side view of finished MCP