# Thin Film Encapsulation for RF MEMS in 5G and Modern Telecommunication Systems
## 1 Introduction  
Radio-frequency microelectromechanical systems (RF MEMS) technology is emerging as a key enabling solution to address the demanding requirements that upcoming fifth generation (5G) standards pose upon passive devices and networks, such as high operating frequencies, large tunability, reduced hardware redundancy, and low power consumption [1; 2; 3]. These performance benefits, however, are offset by a lack of low-cost packaging available for RF MEMS. The package must protect the device from structural damage and contaminants, introduce minimal RF losses, and not degrade the performance of the switch, circuit or complex passive device. To enable widespread implementation of RF MEMS, the package ideally should also be low cost, require little additional space on the wafer, and be easy to incorporate in microwave integrated circuits. MEMS switches can be packaged using wafer-level techniques, in order to avoid the high costs and possible damages due to individual handling and release. Wafer-level bonding and thin film encapsulation are the most common wafer-level packaging methods. In the first case, a ring is applied around the switch and a capping wafer is sealed by solder, glass frit, or friction [4; 5]. The bonding ring causes a significant increase in the device footprint, and the capping wafer may create a high aspect ratio device. Metal-based sealing with solder or Au friction bonding is performed at low temperatures with the possible introduction of high RF losses. Glass-based sealing adds minimal RF loss, but typically requires high temperatures that may damage the switch. By comparison, thin film encapsulation schemes offer a low profile and wafer-level integrated-circuit compatibility. Thin film encapsulation typically uses a sacrificial layer to cover the structures to be sealed, followed by a cap film deposition. The sacrificial layer is subsequently removed by etching through access holes or by thermal decomposition. Finally, a sealing film is deposited over the cap film to seal the access holes. The choice of the material for the encapsulating film is an important concern. With this regard, several materials (organic, metals, and dielectrics) have been proposed [6; 7; 8; 9; 10; 11; 12] with thicknesses in the range of 1-20 \(\upmu\)m. Among the various materials, aluminum nitride, amorphous silicon, and silicon nitride are the most used [7; 9; 10; 11; 12] for thin encapsulating layers. In fact, they satisfy some essential requirements to make the packaging process suitable for industrial production, these include: a good structural integrity and selectivity with respect to the sacrificial material during the chemical etching; an excellent insulation during device operation; and an optimized deposition rate in relation to the thickness to achieve. In particular, mixed frequency plasma-enhanced chemical vapor deposition (PECVD) provides low stress silicon nitride layers that are stable if processed at high temperature [13]. Another crucial issue of thin film encapsulation is that the etch times required to remove the sacrificial material can be excessively long, especially as the size of encapsulation becomes larger, limiting the manufacturability of the package. The long release time may limit the choices of cap and sacrificial layer materials, due to the possible damage of cap layer and/or encapsulated device during the release process. Etch holes distributed over the cap layer [10; 12] or fabricated in its sidewall [12] with standard surface-micromachining techniques can alleviate this problem. Anisotropic plasma assisted etching processes, based on fluorine chemistry, are generally used in the reactive ion etching (RIE) or inductively coupled plasma (ICP) configurations to pattern the encapsulation caps with etch holes. The drawback of RIE and ICP methods is that the substrate bias and physical ion bombardment can modify the stress distribution in encapsulated suspended beams, due to device heating with the consequent beam deformation [14].