# Electropolishing--A Practical Method for Accessing Voids in Metal Films for Analyses
## 4 Results and Discussion  
In the following, a laterally resolved degradation void study is presented, and based on it, the advantages of electropolishing in the preparation of thin films are demonstrated. For the present study, two poly-heaters of different designs are used (Section 2). They are identical in copper, but differ in the functional layers, amongst others, in the polycrystalline silicon that is utilized for heating. During a pulse the central Cu plate of the poly-heater of the first design is homogeneously heated, whereas the one of second design is inhomogeneously heated along its length, being cooler at the center than at the edges. Respective temperature distributions in the Cu at the end of a heat pulse are visualized by the 2-dimensional finite element simulations [65] in Figure 5a,b. Figure 4: (**a**) Cross-sectional schematic illustration visualizing the procedure of accessing voids in the interior of a Cu metallization by material removal during electropolishing. (**b**) Top view SEM image of a thermo-mechanically stressed Cu metallization that has been prepared by electropolishing (2.0 V for 360 s, \(\approx\)5 m material removal) for void analysis. Figure 5: (**a**) Cross-sectional temperature distribution in the Cu layer of (**a**) a homogeneously heated and (**b**) an inhomogeneously heated poly-heater, simulated by finite element method [65]. (**c**) Profile of void area fraction along the lateral direction of the investigated poly-heaters’ Cu plate. After thermo-mechanical stress testing and electropolishing (Sections 2 and 3) the 700 \(\upmu\)m \(\times\) 490 \(\upmu\)m large central Cu plate (Figure 1a) of the two poly-heaters was imaged by a grid of 16 \(\times\) 6 SEM images. These micrographs had the same magnification as the representative one shown in Figure 4b. Subsequently, the voids have been extracted using a digital image processing routine [66]. To prevent pixel noise in the SEM images from being erroneously attributed to as voids, a size threshold has been used, by which only objects with an equivalent circular diameter larger than 60 nm are considered in the evaluation. For the individual images the void area fraction, functioning as a parameter for early microstructural degradation, has been computed by dividing the void-represented area by the total imaged area and saved according to the grid of images. The column-wise statistically evaluated data, corresponding to the laterally resolved profile of the void area fraction across the 700-\(\upmu\)m-long Cu plate, is presented in Figure 5c. One can see that the homogeneously and the inhomogeneously heated poly-heater are characterized by a different distribution of voids. Regions with higher temperature show more pronounced degradation in terms of void area fraction. The slight asymmetry in the data in Figure 5c is very likely to be caused by an asymmetry in the functional layers of the chip. An additional buffer layer (encasing the temperature sensor) extends over an area of 250 \(\upmu\)m \(\times\) 34 \(\upmu\)m below the left half of the Cu plate. This localized layer was not taken into account in the finite element simulations in Figure 5a,b and would lead to slightly decreased temperatures in the left half of the plate. When discussing about electropolishing as a preparation technique for void analyses in thin films, its major advantage compared to cross-sectional preparation is very clear. A large area for statistically relevant void analyses can be accessed that is not constrained by the film thickness. In case one would want to make a similar study as the one presented in this section on mechanically cross-sectioned specimens and achieve the same statistics, the required effort would be substantially higher. Multiple iterations consisting of grinding, polishing, and SEM imaging would be necessary in order to access a total area that is as large as the one of the electropolished specimen.