*3.3. Evaluation of Paste Inks*

3.3.1. Use of Ethyl Cellulose and PGPTMS as a Binder and an Adhesion Reagent, respectively, to Produce a Copper Nitride Paste Ink

Although copper nitride was found to have a high copper conversion ratio, some of the films produced had a high sheet resistance because of weak mechanical strength. To address this issue, and to examine a more practical formulation for copper nitride ink, we investigated the use of a paste ink in which ethyl cellulose and poly(3-glycidoxypropyl)trimethoxysilane (PGPTMS) were used as a binder and adhesion reagent, respectively. It was previously reported that PGPTMS improves bonding strength between the particles in ink films and between the substrate and the film [28,29].

First, we determined the most suitable concentration of PGPTMS to use in the vehicle by preparing vehicles containing different concentrations of PGPTMS and using them to make films containing copper nitride at a weight ratio of 1:1 (Table 3). The films were then exposed to IPL irradiation under different conditions (Table 4), and copper conversion ratio and sheet resistance were measured (Table 5, Figure 5, and Figure S5).



**Table 4.** Intense pulsed light sintering conditions.

**Table 5.** Copper conversion ratio and sheet resistance obtained by using the intense pulsed light sintering conditions shown in Table 4.


O.L. = overload.

**Figure 5.** Conversion ratio (**a**) and sheet resistance (**b**) of films made with various vehicles (Table 3) and then exposed to intense pulsed light irradiation using the conditions shown in Table 4.

Among the inks containing vehicles 1, 2, and 3 (PGPTMS concentrations, 0, 1, and 7 wt %, respectively), the ink containing vehicle 2 had a high copper conversion rate under most conditions. We therefore examined the effect of different IPL irradiation conditions on this ink. In a comparison of different applied voltages (2.0 [P.S.1] or 2.3 kV [P.S.2]), the copper conversion rate was increased from 0.43 to 0.63 by increasing the applied voltage. In a comparison of different pulse widths (1000 [P.S.2], 1500 [P.S.3], or 2000 μs [P.S.4]), the copper conversion rate increased with increasing pulse width, although the copper conversion rate was comparable between pulse widths of 1500 and 2000 μs. In a comparison of number of pulses (1 [P.S.2] or 4 pulses [P.S.5]), the copper conversion rate was increased from 0.63 to 0.75 by increasing the number of pulses. In addition, low sheet resistances, 5.06 × <sup>10</sup>−<sup>1</sup> and 4.97 × <sup>10</sup>−<sup>1</sup> <sup>Ω</sup> sq−1, were obtained for conditions P.S.2 and P.S.5, respectively. No relationship was observed between copper conversion rate and sheet resistance.

To examine the relationship between sheet resistance and bonding between particles, the surface condition of the sintered samples prepared by using vehicle 2 was observed by means of scanning electron microscopy (Figure 6). Improved necking and no increase in particle size above micrometer size were observed in the film irradiated using the P.S.2 conditions (8.30 J cm−2). In addition, the influence of the additives (i.e., ethyl cellulose and PGPTMS) could be clearly observed between the paste inks (Figure 6) and the liquid inks (Figure 4). The additives inhibited the intense reaction of copper nitride by IPL irradiation (described above in the Section 3.2) because the heat energy produced by IPL irradiation spread not only to the copper nitride, but also to the additives. As a result, the formation of the particles above micrometer size with a hollow shape was inhibited. Furthermore, it appears that the necking between particles was improved by the addition of the additives because the sheet resistance was decreased and mechanical strength was increased in films containing the additives compared with those without.

In this series of experiments, the copper conversion ratio was limited to approximately 0.8 for the paste ink, owing to the penetration depth of the IPL irradiation. For all of the examined films, there was a difference in color after IPL irradiation when both sides of the film were compared (Figure S6); the back of the film was darker than the front, indicating that the copper nitride was not converted to copper at the back of the film.

**Figure 6.** Scanning electron microscopy images of films prepared by using vehicle 2 before and after intense pulsed light sintering with different irradiation conditions.
