# Thin Film Encapsulation for RF MEMS in 5G and Modern Telecommunication Systems
## 4 Release as a Function of Cap Holes  
Surface profilometry and SEM investigations were combined to evaluate the removal of the sacrificial layer as a function of the percentage of cap perforated area as well as the mechanical robustness of the fabricated caps. Figure 3a-d shows caps with different hole matrixes, which were released after the same barrel etching with t = 8 min and P = 400 W. For A = 26%, a residual sacrificial layer was observed under the central part of the cap (Figure 3a). By increasing A up to 50%, the residual sacrificial layer reduces, but does not disappear (Figure 3b). The complete release of the cap is observed at A = 57% (Figure 3c) and A = 74% (Figure 3d); the highest A value corresponds to the largest perforated area that can be etched without the cap collapsing on the CPW. SEM analysis was performed for caps released with different powers and times of the barrel etching. It was found that similar results were obtained for t = 8 min, whereas the sacrificial layer was completely removed even for A = 26% when t = 20 min and P > 300 W or t = 80 min and P > 100 W were used.  
Figure 3: Tilted SEM images (tilt angle of 60\({}^{\circ}\)) of the fabricated encapsulating caps with A = 26% (**a**), 50% (**b**), 57% (**c**), and 74% (**d**). Profiles measured for caps in (**a–d**) under the application of a stylus tip force of 4.9 \(\mu\)N and 490 \(\mu\)N (**e**). Deflection in the center of the caps in (**a–d**) as a function of the stylus tip force (**f**). The marker is the same for all SEM images. The green line in (a) is the profiler scan line. Barrel parameters, which were used for the release of caps in (**a–d**), are P (power) = 600 W and t = 8 min.  
Linear profiles along the caps in Figure 3a-d were performed under the application of different stylus tip forces. Figure 3e shows the comparison between the scans along caps under the application of the lowest (4.9 \(\upmu\)N) and the highest (490 \(\upmu\)N) force values. In the case of the lowest force, the profile is approximately the same for caps with A \(<\) 57%, while a lowering and a flattening are observed for the cap with A = 74%. When the highest force value of 490 \(\upmu\)N is applied, the caps tend to collapse on the CPW; that is, they bend up to touch the CPW, with possible evidence of residual sacrificial material. Consistent with SEM images, the collapse of the cap on the CPW is not complete for A = 26% and A = 50% since a feature of a few microns was observed in the central part of the cap. The height of this feature reduced from 5 \(\upmu\)m to 2 \(\upmu\)m as A increased from 26% to 50%. For greater values of A, the cap was completely collapsed on the central conductor of the CPW with no evidence of residual sacrificial material. In all cases, the cap results were stiffer at the anchoring points than in the internal region, as already observed in another study [10]. Figure 3f shows how the center of caps with the various A values deflects as a function of the stylus tip force. It is observable that the cap with A = 26% shows a deflection for forces greater than 100 \(\upmu\)N. In particular, it deflects by \(\sim\)30% under the application of 196 \(\upmu\)N and even more when the applied force is increased, until the maximum deflection for F = 490 \(\upmu\)N when the cap collapsed on the residual sacrificial material. With increasing A from 26% to 50%, the cap bends for F = 98 \(\upmu\)N with a deflection of 2 \(\upmu\)m. Flexibility further increases for the cap with A = 57%. The cap with A = 74% collapses on the CPW for a force as low as 49 \(\upmu\)N, however no breakage is observed for forces up to 490 \(\upmu\)N.