Published online by Cambridge University Press: 05 February 2014
Introduction
MEMS devices, unlike conventional ICs, contain movable fragile parts that must be packaged in a clean and stable environment. The package or encapsulation should not only offer protection to the MEMS during operation but also during fabrication. The specific ambient (i.e. gas composition and pressure) of the package housing depends on the type of RF MEMS. For instance, switches are preferably housed in an inert ambient (e.g. dry nitrogen) at, or slightly below, atmospheric pressure. The same applies to RF MEMS variable capacitors and tuneable inductors. Micromechanical resonators, on the other hand, require a high level of vacuum (e.g. ambient pressure <1 Pa) to attain high-frequency stability and to have sufficiently low damping at resonance. Practically, all MEMS are adversely affected by corrosive ambients like moisture. To ensure stability of the MEMS, the package must offer hermetic (or near-hermetic) seals. Sealing and encapsulation are crucial to provide the required reliability of the packaged devices.
Essentially, two approaches for device encapsulation can be defined. Encapsulation can be accomplished using conventional 1-level ceramic (e.g. LTCC), AlN or metal canister (usually abbreviated to ‘can’) packages [1]. The 1-level package consists of the chip’s capsule and associated leads for interconnecting the chip to the outside world [1]. For ceramic packages, encapsulation can be achieved by soldering or brazing a ceramic cap or lid to a metal sealing ring on the substrate, thus defining the cavity housing of the MEMS device [1–4]. Metal hermetic packages are commonly welded, soldered or brazed [1, 4, 5]. Cavity formation during 1-level packaging is an established method and allows a certain flexibility with respect to the composition of sealing gas and pressure, but ceramic or metal packages are expensive. The high cost of 1-level packaging is viable for telecommunications base stations, satellites and defence systems, but not for high-volume applications like mobile phone handsets. Furthermore, 1-level packaging poses technological complications – mainly due to handling of the MEMS after their release. For instance, the standard wafer sawing or the injection moulding process of plastic packages cannot be used, because it may destroy or contaminate the released MEMS device. Once the wafer is diced, the MEMS chips must be handled in an extremely clean environment, because cleaning in a liquid is no longer possible at this stage. All this suggests that packaging is preferably carried out during wafer processing, prior to die singulation.
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