# Investigation of the Influence of Sublayer Thickness on Pairing of Metallic MEMS Shutter Blades
## 3 Fabrication Technology  
The standard fabrication process for paired micro-shutters has been explained in detail previously [16], and our understanding is improved with further investigations on shutter pairing [29]. Here, a summary of the conducted steps in the process is provided: (a) the cleaning and preparing a glass substrate coated with a 650 nm thick layer of Fluorine-doped Tin Oxide (FTO), which serves as a bottom electrode, (b) the deposition of a 1000 nm thick isolation layer of silicon dioxide (SiO\({}_{2}\)) using the Plasma Enhanced  
Figure 3: Two fundamentally different geometries of the shutter blades: (**left**) unpaired with different optional curling states and (**right**) pairing of curled shutter blades A and B. The colour code used is explained in Section 4. Chemical Vapour Deposition (PECVD) technology, which serves as an isolation layer between the bottom and top electrodes, permitting the electrostatic actuation of the fabricated micro-shutters, (c) the optical lithography process is used to pattern the structure of the shutter blades, (d) the deposition of a three-layered metal stack using electron beam Physical Vapour Deposition (PVD), with thicknesses of \(d_{\mathrm{Al1}}\) = 99 nm, \(d_{\mathrm{Cr}}\) = 16 nm, and \(d_{\mathrm{Al2}}\) = 88 nm, (e) the sacrificial layer of photoresist is dissolved in N-Methyl-2-Pyrrolidone (NMP), and (f) an we perform adapted drying process of the fabricated sample that promotes the shutter blades' pairing. In the drying process that is depicted in Figure 4. (refer to Supplementary Materials Figure S2 for an image of the setup), the sample is immersed in isopropanol at a 25\({}^{\circ}\) tilt angle and covered with a steam bell, enabling the sample to dry in a gaseous environment. The drying setup is placed on a hotplate at 90 \({}^{\circ}\)C resulting in a 60 \({}^{\circ}\)C liquid bath. The liquid slowly evaporates and creates high vapour pressure inside the steam bell, pushing out the liquid of the steam bell into the outer tank, and later releasing it from the tank. A thin film of liquid is formed at the sample's surface. Due to the gradual movement of the thin liquid film (row by row) and by means of the liquid surface tension, shutter A moves back, meanwhile, shutter B is still in a curling state. The combination of shutter B's curling state and shutter A's movement determines whether the shutter blades will pair or not. (Refer to the video in Supplementary Materials for a visual explanation.) This specific interplay of the two blades also determines which one of the observed three pairing scenarios occurs (for details, see the Section 4). We must bear in mind that the initial curling state is mainly affected by the multilayer metal stack comprising the shutter blades. The pairing phenomenon was first observed in our study at \(d_{\mathrm{Al2}}\) = 88 nm [16]. Among the others, the pairing the two blades is used to study the role of attractive Casimir forces. For the Casimir force, the interacting material is only Al. The change in the stressed metal layer thicknesses results in different RoCs. This can be observed via changing \(d_{\mathrm{Al2}}\) or \(d_{\mathrm{Cr}}\) or via changing both of them. To obtain clear results, the number of parameters changed was kept at a minimum. Therefore, only \(d_{\mathrm{Al2}}\) was changed. The process was repeated to ensure its reproducibility and stability. The produced samples have revealed the uniformity of curling. Within this investigation, the thickness of \(d_{\mathrm{Al2}}\) was varied, and we chose it to be 55, 66, 70, 75, 80, 84, 88, and 92 nm. We should bear in mind that the dimensions of one sample containing more than 150,000 identical single micro shutters are 10 \(\times\) 10 cm\({}^{2}\). Figure 4: The drying setup placed on a hotplate consisting of a glass tank with a dispenser and steam bell. The sample holder is placed inside, and this allows us to use differently angled positions. The gradual movement of the isopropanol liquid and forming of the paired shutters are shown in (**a–c**).