# Thin Film Encapsulation for RF MEMS in 5G and Modern Telecommunication Systems
## 2 Outlook of 5G and RF Passives Requirements  
Across the last four decades, there has been a seamless spread and evolution of mobile communication devices and services, starting from the first cellular network generation (1G) during the 1970s and 1980s, to the current fourth generation Long Term Evolution (4G-LTE), deployed in 2010 [15; 16; 17; 18; 19; 20; 21; 22]. Given this scenario, the upcoming 5G of mobile communications will step forward along the visionary direction of an all-communicating world [23; 24]. Such a perspective can be inflected in high-level specifications according to the following reference list [25; 26; 27; 28]:  
1. Increase of data volume up to 1000 times;
2. Increase of connected devices from 10 to 100 times;
3. Typical user data rate increased from 10 to 100 times;
4. Extended battery life up to 10 times;
5. End-to-End (E2E) latency down to the milliseconds range. The 5G will have to realize a common connected platform able to satisfy the above-listed specifications. This will require acting at diverse technical levels [29; 30; 31; 32; 33]:  
1. Radio links;
2. Multi-node/multi-antenna transmission;
3. Network dimension;
4. Spectrum usage. Given the system-level specifications discussed above, it is now necessary to frame how they reflect in terms of characteristics and requirements that basic hardware components and RF passives must comply with, in order to sustain and empower the high-level vision of 5G. To this purpose, as the first step it is appropriate to list the classes of RF passive components and networks, which will be necessary for 5G applications. It must also be stressed that all the devices mentioned can be effectively realized in RF MEMS technology [34]:  
1. Wideband switches and switching units with low-loss, high-isolation, very-low adjacent channels cross-talk, working from 2-3 GHz up to 60-70 GHz (and more);
2. Reconfigurable filters with high stopband rejection and low attenuation of the passed band;
3. Wideband multi-state impedance matching tuners;
4. Programmable digital step attenuators with multiple configurations and flat characteristics over 60-70 GHz frequency intervals;
5. Wideband multi-state/analogue phase shifters;
6. Hybrid devices with phase shifting and programmable attenuation (i.e., examples 4 and 5 as a unique device);
7. Miniaturized antennas and arrays of antennas integrated monolithically with one or more of the previous devices. Now that the main macro-classes of RF MEMS passives that 5G will demand are mentioned, it is useful to list the most relevant tentative specifications that such devices will have to meet in order to be successfully employed within 5G-related applications. Such specifications are briefly listed as follows:  
* Frequency range: From sub-6 GHz up to mm-wave range (60-70 GHz and more);
* Isolation: Better than \(-\)30/\(-\)40 dB for frequencies as high as possible;
* Loss: Below \(-\)1 dB on the widest possible frequency range;
* Cross-talk: Below \(-\)50/\(-\)60 dB over the widest frequency range possible;
* Switching time: Lower than 1 ms, with few fractions of us (e.g., 200-30 us) as reasonable target;
* Control voltage: Within a few Volts (e.g., 2-3 V). In light of the just sketched scenario, RF MEMS technology has already demonstrated its ability to address the demanding requirements imposed by 5G on RF passive components, in terms of wide reconfigurability/tunability, wideband operability, and frequency agility [1; 2; 35]. Nonetheless, a crucial aspect to be carefully handled, in order to exploit the full commercial potentialities of RF MEMS, is that of packaging and integration. As a matter of fact, microsystems technology is either incompatible or difficult to blend monolithically with standard semiconductor technologies (e.g., CMOS). As a result, employment of RF MEMS components within systems and sub-systems always leverages integration via surface mount technologies (SMTs), like flip-chip, ball grid arrays (BGAs), wire-bonding, and so on [36].