# Investigation of the Influence of Sublayer Thickness on Pairing of Metallic MEMS Shutter Blades
## 4 Experimental Results and Discussion
### 3D Self-Assembled Paired Shutter Blades (Yin-Yang shapes)
Geometrical changes of the paired shutters are also observed in the overlapping area between the shutter blades, which we describe here as pairing scenarios. The plot shows that samples with \(d_{\text{Al2}}=88\) and 84 nm exhibit two pairing scenarios each. As \(d_{\text{Al2}}\) decreases, the pairing scenario changes for the samples with \(d_{\text{Al2}}=80\), 75, and 70 nm, therefore, a different pairing scenario is observed for each, respectively. The shape of the overlapping area defines the pairing scenario, i.e., how shutters A and B overlap and attach to each other, forming the Yin-Yang geometry. The three pairing scenarios are labelled with roman numbers (I), (II), and (III), and they are displayed in different blue tones in Figure 8 in relation to the eccentricity of shutter B. Scenario (I): shutter A is slightly curling at the top part of the pairing area (Figure 9 (I)). Scenario (II): both of the shutter blades are flat and completely attached inside the overlapping area (Figure 9 (II)). Scenario (III): shutter B curls at the lower part of the overlapping area, coming together with stronger curling on the entire lengths of shutter B (Figure 9 (IIIa)). Figure 9 (IIIb) reveals stronger curling on the entire lengths of shutter B. The different scenarios directly refer to the dynamics of the drying and 3D self-assembling of the shutter blades pairing process. Two videos (Video S1: Dynamics of the drying process) of the drying process for a sample with \(d_{\mathrm{AL2}}=84\) nm have been recorded. The videos support that the initial curling state of the individual shutters strongly influ  
Figure 8: The eccentricity of shutter B is shown as a function of the thickness of the top Al layer (\(d_{\mathrm{AL2}}\)). Figure 9: Illustration of different pairing scenarios: (I) shutter A curls at the top part of the overlapping area, (II) shutter A and B are fully flat inside the overlapping area, (IIIa) shutter B curls at the lower part, and (IIIb) shutter B curls even more. In the centre of the figure is a representation of the measured overlapping length \(l_{o}\), ranging between a maximum of 31 \(\upmu\)m (II) and a minimum of 8 \(\upmu\)m (III), and the stiction length \(l_{s}\) is observed to be 0 \(\upmu\)m for scenario (I) increases up to 90 \(\upmu\)m for scenario (III). The anchor of each shutter is shown in grey. ences the chances of shutter A finding shutter B and pairing with it. As long as shutter A moves back via the liquid surface tension, the curling state of shutter B decides which pairing scenario will occur. Since drying is a sensitive process (involving gaseous and liquid turbulences), for the same thicknesses, different scenarios for \(d_{\text{Al2}}\) of 84 and 88 nm can occur due to the shape of shutter B during the moment of its attachment to shutter A (its RoC). For us to conduct a further investigation of the observed paired scenarios, two measurements were considered. The overlapping length (\(l_{0}\)) is defined as the length of the tight attachment between shutters A and B, and the stiction length (\(l_{s}\)) is the length of shutter A that is sticking to the substrate (exclusive anchor length). See Figure 9 for an intuitive representation. Starting from an average length of 16 \(\upmu\)m for the overlapping in the pairing scenario (I), \(l_{0}\) increases in scenario (II), peaking at 31 \(\upmu\)m (the most significant value observed in this investigation at \(d_{\text{Al2}}=88\) nm). Finally, it decreases again, reaching an observed minimum of 8 \(\upmu\)m in scenario (III) at \(d_{\text{Al2}}=70\) nm. The resultant values are in accordance with the reported RoCs and eccentricities, since more pronounced curling reveals larger intrinsic stress, thereby it does not allow flattened overlapping to occur between the two shutters. Nevertheless, the point of the blades' touch during the pairing process also affects the overlapping, as scenario (I) depicts.