# Electropolishing--A Practical Method for Accessing Voids in Metal Films for Analyses
## 3 Electropolishing Setup  
A schematic representation of the setup used for electropolishing (a 2-electrode setup) is shown in Figure 1b. This rather simple setup comprises a DC voltage supply, a beaker, in which the two electrodes are in electrical contact with each other via an electrolyte, and a magnetic stirring system. The specific electrolyte used is well-suited for electropolishing copper and is composed of 45 vol% orthophosphoric acid (concentration 85%), 45 vol% methanol, and 10 vol% isopropanol. For the electropolishing experiments presented in this work the poly-heater specimens were electrically contacted by an alligator clip, as shown in Figure 1c. To allow electropolishing of the central Cu plate without damaging it, the alligator clip was used to contact one of the measurement pads that are internally connected to the central plate. By using an alligator clip made of copper, no additional metal was introduced to the system, minimizing measurement errors due to the formation of a galvanic coupling and possible galvanic corrosion of the chip's electropolished surface [52; 53]. As depicted in Figure 1b, the alligator clip clamping the poly-heater to be electropolished is fully immersed into the electrolyte and acts as an anode. By this means, the contact surface between the anode and the electrolyte, determined by the electrically contacted Cu surface of the poly-heater (\(\approx\)2 mm\({}^{2}\)) and the alligator clip (\(\approx\)400 mm\({}^{2}\)), becomes large enough such that the resulting electrical currents can be measured, and reproducibility of the operating conditions can be guaranteed. A cylindrically shaped mesh grid made of stainless steel with height 42 mm and diameter 16 mm functions as a cathode. All experiments were performed at room temperature and with stirring of a frequency of 200 revolutions per minute. In addition to the electrode materials and the electrolyte, multiple parameters influence the electropolishing process. These comprise electrode distance, surface area ratio of the cathode with respect to the anode but also applied voltage (and corresponding current density), electropolishing duration, stirring, temperature, electrolyte bath age, etc. [33; 38; 39; 44; 54; 55]. [PERSON] et al. [40] recommend to acquire a current-voltage curve to find the optimal electropolishing voltage for the particular setup and operating conditions in use. Such a curve characterizing the setup in the present publication is shown in Figure 2. Well in agreement with reports in literature [29; 32; 44; 46; 56; 57], three different regimes can be identified: the first regime ranges from 0.0 V to roughly 0.7 V and is determined by an increasing current. In this regime, etching of the metal (preferentially at grain boundaries) occurs and for this reason no surface smoothing takes place [30; 40]. The second regime (extending from 0.7 V to 2.5 V) is characterized by a constant current density  
Figure 1: **(a)** Light microscopy image of a poly-heater. **(b)** Schematic diagram of the electropolishing setup and **(c)** an alligator clip electrically contacting a poly-heater for electropolishing. and denotes the interval, in which the optimal voltage in terms of surface smoothing can usually be found [49; 56; 57; 58]. In this regime, a viscous liquid layer is formed on the anode's surface, where the continuously dissolving Cu is present at high concentrations in form of aqueous cupric salts. Their transport away from the anode's surface is limited by diffusion processes that manifest in the constant current [29; 48; 49]. The principle leading to surface smoothening can be explained, as follows. Projections of the anode's surface protrude further into the viscous liquid layer than crevices and for this reason they reach out to regions with lower ionic concentrations. This causes a more effective dissolution and faster material removal of the projections, leading to a gradual levelling of the surface.