# High-Resolution Printing of Various Electronic Materials by Electrophotography
## 3 Results and Discussion  
Figure 3 displays an SEM image of the printed microcircuit pattern obtained via toner printing. The bright pattern comprises Ag-Ni conductive particles, and the dark background corresponds to the carbon tape used for SEM observation. The Ag-Ni pattern was directly transferred from the developed substrate surface to the carbon tape surface. The right side of the SEM micrograph shows an Ag-Ni interdigital electrode pattern with a linewidth of 30 \(\upmu\)m, and the left side shows a 10 \(\upmu\)m linewidth interdigital electrode pattern printed using the electrophotography method. The 30 \(\upmu\)m linewidth interdigital electrode patterns show a precise contour similar to their designed original shape and comprise sufficiently uniform and dense Ag-Ni particles that can be used as electrode patterns in microelectronic circuits. The 10 \(\upmu\)m linewidth pattern also exhibits a clear shape, as originally designed; however, the contour of the 10 \(\upmu\)m linewidth pattern is slightly jagged. The success rate of printing 5 \(\upmu\)m linewidth interdigital electrode patterns is currently relatively low and depends on the material, i.e., the particle diameter and fluidity of the powder. Among the present toner particles, Ag-Ni and indium tin oxide (ITO) are suitable for the 5 \(\upmu\)m pattern. Figure 2: (a) Developing system; (b) photograph of magnetic brush. The energy dispersive X-ray (EDX) spectrum of the printed Ag-Ni toner pattern transferred to the surface of the carbon tape is provided in Figure 4. Characteristic X-ray lines of Ag, Ni, Fe, C, and O can be observed from the Ag-Ni toner pattern surface. Ag lines appear at approximately 3 keV as the main peak, and slight Fe and Ni lines are apparent at approximately the 6.3 and 7.5 keV regions, respectively. Owing to the atomic ratio calculation, excluding O and C, 94% Ag, 1.8% Ni, and 3.6% Fe were estimated. As the nominal mixing atomic ratio of Ag:Ni was 5.44:1, the expected Ni ratio was 15.5%. However, the Ni atomic ratio in the printed Ag-Ni pattern was remarkably lower than the nominal ratio. This is because the magnetic field attracts excess Ni nanoparticles that remain in the developer's powder. Consequently, the purity of Ag in the printed Ag pattern becomes higher than that in the nominal pattern, which effectively prevents an increase in the electrical resistivity, regardless of the addition of Ni nanoparticles. Therefore, the Ni addition effectively improved the fluidity of the Ag toner particles and did not degrade the electrical conductance of the printed Ag pattern. In addition, the slight amount of Fe detected by EDX can be attributed to small fragments of the carrier particles. The weight ratio of the Fe components is less than 1%. However, a higher Ag ratio is crucial for approaching the lower limit of electrical resistivity. Figure 5 shows the optical micrographs of the printed micro-interdigital electrode pattern obtained using various electronic materials. Ag-Ni in Figure 5a and Cu NPs in Figure 5b appear comparable. Ni NP-added Ag NPs are superior to Ag NPs in terms of fluidity [85]. Therefore, the results printed with Ag-Ni NPs are more precise and stable, particularly in high-resolution printing. Cu NPs in Figure 5b and Zn NPs also show a 10 \(\upmu\)m linewidth scale resolution. The Cu and Zn NPs exhibited high fluidity, probably because of surface oxidation of Cu and Zn. However, Au particles have a highly active surface; therefore, they spontaneously grow into larger particles over time by taking the surrounding Au particles onto their surface. More specifically, Au nanoparticles cannot exist as dried nanoparticles. In contrast, Au solution inks contain chemical agents that separate the Au particles; therefore, they do not directly contact the Au surfaces in the solution. Consequently, creating a high-resolution (10 \(\upmu\)m scale) Au pattern using electrophotographic printing is currently difficult, although macro-scale Au circuit printing is possible.