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MEMS Packages and Applications

Chapter 12 MEMS Packaging Materials

Ann Garrison Darrin and Robert Osiander

Abstract.This chapter discusses the differences between the heritage microcircuit packaging world and the still evolving MEMS packaging arena. Materials used in the packaging of MEMs are reviewed and their respective applications. The packaging schemes for these devices owe their infrastructure base to the body of knowledge surrounding semiconductors and microcircuits. MEMS devices yield new complexities which drive new packaging solutions. As opposed to traditional microcircuit chips, MEMS often include moving structures along with the need to have contact with the external environment, driving the requirements for packaging such components. Thus, some functions include interaction with the surrounding environment, such as pressure sensors. This imposes new requisites on packaging, since in regular microelectronics the chip must be protected completely from any impact from the environment. Packaging also provides mechanical support to the sensitive chip, facilitating the handling of the chip, and simplifying assembly. This chapter emphasizes the materials involved in packaging MEMS devices and the addresses the challenges involved. Included in this chapter are several case studies demonstrating novel packaging/material solutions.

MEMS Packages and Applications

Packaging for MEMs provides for power and signal paths; thermal management; mechanical support; and environmental protection and/or interface protection. MEMS pose numerous packaging challenges that often drive custom solutions by the very nature of their configurations and applications. Commercialized MEMS products such as inkjets and airbag sensors use simple packaging solutions; but devices that involve environmental interface have special requirements that drive novel approaches (i.e., bio-MEMS packages that need to handle fluids). MEMS packaging can be roughly divided into two areas: protection at the package level and protection at the wafer or device level. At the package level, MEMS are often manufactured following traditional military specifications that require robust hermetic sealing technologies. This branch of the packaging industry transferred almost intact from the semiconductor/microelectronics industry. However, this approach is in rapid decline with the main trend being away from military specifications and their high reliability electronics toward less expensive commercial, off-the-shelf (COTS) electronics. The wafer-level packaging and protection of MEMS structures enable low cost, high volume production packages that are not hermetically sealed; e.g., plastic packages or tailored RF packages with optimized electrical performance. The fact that MEMS often must be in contact with the operating environment is the most important distinction between packaging of MEMS and microelectronics.

MEMS packaging protects the mechanical structures during wafer dicing, assembly, and operation [1]. For example, during the singulation step, a mask or a wafer-level technique is used to protect vulnerable motion components as the most common MEMS packaging requirement is protection without restricting mechanical action in a cavity device. Microcircuit packaging techniques are often over molded with encapsulant contacting the chips and restricting motion and cannot be used as a MEMS packaging technique without redesign.



The hermetic cavity package used extensively in the microcircuit world, especially in critical end item applications, has been adopted for the packaging of MEMS. These standard hermetic packages are a costly solution but do offer full hermetic isolation. With respect to cost, a prevailing opinion has been that MEMS packaging is expensive. This has caused developers to question the application and the requirements, especially when hermetic packaging is specified. For example, is hermetic packaging truly required or just a dry, hydrophobic package or perhaps only a constant atmosphere is needed [1]? Early MEMS accelerometer packages were hermetic cavity designs that allowed space for motion, although these ceramic packages have since been replaced with more cost effective designs. Inertial sensor applications require only electrical inputs and outputs and do not have the packaging challenges such as in bio-MEMS where fluid transport is incorporated. MEMS packaging options range from full hermetic, near-hermetic, and non-hermetic single packages to wafer-level packaging techniques. The materials for each of these packaging classes are discussed.


Date: 2015-02-28; view: 1070


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