# Icing Mitigation by MEMS-Fabricated Surface Dielectric Barrier Discharge
## 2 SDBD Preparation  
### MEMS Fabrication  
MEMS technology enables an implementation of the desired geometry with sub-micrometer accuracy. The MEMS manufacturing steps and principles are shown in Figure 1. In a first step, the substrates were pre-cleaned in an isopropanol and an acetone ultrasonic bath for 10 min. Then the 4-inch substrates were spin coated at 4000 rpm with an AZ 5214 NV image reversal photoresist and soft-baked for 60 s at 100 \({}^{\circ}\)C. The electrode pattern was structured by optical lithography (365 nm) and after the reversal image procedure the substrates were developed in AZ 726 MIF to remove soluble photoresist areas. In a fourth step the samples were metallized by an electron beam physical vapor deposition (EB-PVD), with which the electrode thickness can be adjusted accurately to a few nanometers. A lift-off process in acetone removes the AZ 5214 NV and finalizes the fabrication process of one electrode side. The counter electrode was produced analogously after turning the sample over. Alignment marks on the sample ensure a precise counter-electrode positioning. An erosion study was conducted to evaluate an appropriate electrode material. For this purpose, three SDBDs were prepared on a borofloat glass substrate for each electrode material to be investigated. The SDBDs were then operated slightly above the ignition voltage for 30 and 60 min, respectively. Scanning electron microscope (SEM) images of the non-operated and the two eroded samples are presented in Figure 2. The high-voltage electrode of the SDBD is colored in false-color in each picture. The images were taken with a JEOL JSM-6510 at an inclination angle of 60\({}^{\circ}\) after sputtering a few nanometers of gold onto the samples to avoid insulator charging effects and, thus, to improve the image quality. Note that the supplementary provides the original SEM photos at a higher resolution (Supplementary S1-S5). Figure 1: Principles and technological steps of the MEMS-manufacturing process. Figure 2: Erosion study to find a suitable material for the SDBD electrodes. The SEM images of the MEMS-fabricated SDBD electrodes taking at an inclination angle of \(\alpha=60^{\circ}\) and displayed in false color. The left column presents the electrodes directly after the fabrication with indication of their materials or material combinations. The other columns shows the electrodes after SDBD operation by a \(V_{rms}=2.2\,\mathrm{kV}\) sine wave with a frequency of \(f=1\,\mathrm{kHz}\) during the time displayed in the column’s index. A chromium (Cr) layer provides an excellent adhesion to the glass surface, however, it erodes quickly under air plasma operation, as it is shown in Figure 2. Using Cr as adhesion promoter a second metal layer can be used to protect the electrodes from a fast erosion. Since the erosion of MEMS-fabricated SDBDs [35; 36] is of extraordinary importance for the durability of the device, erosion studies can already be found in the literature [16; 32; 33; 37]. It was shown that tungsten (W) [16; 33; 37] has a good resistance against the destructive forces of the low-temperature plasma. In [32], we reported on the erosion of the commonly used Cr/Cu electrodes and also showed that a protective layer of gold (Au) restricts the erosion only to a certain degree. In the same study Cr/Ni electrodes [32] were found to be the most suitable electrode metals in terms of erosion. The present study now compares the Cr/Ni electrode erosion to the tungsten electrode erosion. Figure 2 exhibits that Cr/Ni depicts a better erosion resistance than tungsten, however, the additionally analyzed titanium (Ti) electrodes revealed the least signs of erosion. Note that titanium was already used by [PERSON] et al. [16], but only as a 10 nm thick adhesion promoter in a multi-layer high-voltage electrode composed mainly of tungsten. They also propose a TiN layer coating since it is a well-known wear resistant, however, it was not evaluated whether this protective layer really diminishes the erosion.