# Thin Film Encapsulation for RF MEMS in 5G and Modern Telecommunication Systems
## 3 Fabrication Flow Chart  
Figure 1 shows a sketch of the process flow for the fabrication of the thin film encapsulating caps. All samples were fabricated on 3 in, ISO standard silicon wafers, covered with 500-nm thick silicon oxide. A 450-nm thick aluminum layer was deposited at room temperature by RF sputtering and patterned by optical lithography and lift-off techniques to define CPWs (Figure 1a). The positive resist SPR7.0 was used with a thickness of 8.5 \(\upmu\)m as the sacrificial layer to define the air gap areas under caps on the central conductor of CPWs (Figure 1b). Thin films in the trilayer SiN\({}_{\text{x}}\)/a-Si/SiN\({}_{\text{x}}\) with thicknesses of 220/350/227 nm respectively, were deposited in a single process using a coupled planar parallel electrode Multiplex Series PECVD (Surface Technology Systems, Ltd) with an electrode diameter of 24 cm (Figure 1c). The system was equipped with a high frequency (HF) generator at 13.56 MHz and a low frequency (LF) generator at 380 kHz. The operating temperature of the shower head of the machine was 200 \({}^{\circ}\)C, while the temperature of the chuck where the wafer was placed during the deposition process was 150 \({}^{\circ}\)C. The other parameters for nitride depositions were: 40 sccm of NH\({}_{3}\), 40 sccm of SiH\({}_{4}\), 1960 sccm of N\({}_{2}\), chamber pressure of 900 mTorr, HF generator power of 30 W, LF generator power of 20 W, HF/LF cycle time of 8 s/2 s. The parameters for the deposition of a-Si were: 20 sccm of SiH\({}_{4}\), 1000 sccm of Ar\({}_{2}\), chamber pressure of 1000 mTorr, LF generator power of 50 W, process time of 9 min. Stress measurements, using the Stoney wafer curvature variation equation [43], were performed with a profilometer KLA-Tencor P6 surface profiler, equipped with a 2-\(\upmu\)m diamond stylus tip. Stress measurements were carried out on layers of SiN\({}_{\mathrm{x}}\), a-Si, and SiN\({}_{\mathrm{x}}\)/a-Si/SiN\({}_{\mathrm{x}}\) directly deposited on the wafer in order to achieve an indication of the stress inside the suspended caps that will be fabricated with these material layers. Tensile stress values of 14 MPa, 12.6 MPa, and 24 MPa were measured for SiN\({}_{\mathrm{x}}\), a-Si, and SiN\({}_{\mathrm{x}}\)/a-Si/SiN\({}_{\mathrm{x}}\) layers, respectively. The stress measurement for the trilayer is greater than the expected value, which is the weighted volumetric average of the single layers. A possible reason for this result is that the first nitride layer undergoes an increase of the residual stress during the depositions of the amorphous silicon and the second nitride layers, due to the heating and subsequent H outgassing [44]. Three profiles were measured for each sample along its diameter; the resulting stress data show a standard deviation better than 5%. Two barrel etchings were performed in the same chamber in order to create a matrix of holes through the SiN\({}_{\mathrm{x}}\)/a-Si/SiN\({}_{\mathrm{x}}\) caps (Figure 1d) and to remove the sacrificial layer (Figure 1e). The barrel parameters used for the two etching processes are summarized in Table 1. For the patterning of top and bottom SiN\({}_{\mathrm{x}}\) layers, an etching time of 60 s and 90 s was used, respectively. Power and time  
barrel etcher. For comparison, the same hole obtained changing the barrel etcher with an ICP etcher is shown in Figure 2e. It is notable that the trilayer structure of the capping layer is more evident in the case of the hole etched in the barrel reactor, due to the inclined sidewalls, as already observed in the cross sectional analysis. Furthermore, the diameter of the barrel hole is about 30% larger than the ICP hole. Both these effects are due to the isotropic characteristic of the barrel etching. Fabricated caps show a concave shape with a height of \(\sim\)15 \(\upmu\)m (Figure 2f).