Green laser sintering of copper oxide (CuO) nano particle (NP) film to form Cu conductive lines

In the last decade, direct printing methods have been used in the field of printed electronics, due to their advantages of fast, cheap, simple and cost-effective process.^1–7 Among others, metal NP ink has been used to obtain conductive patterning via printing. After printing of metal-based NP ink, a sintering process is required to obtain the desired conductivity. Recently, Cu NP ink has been developed as a low cost alternative to Au and Ag NP ink.^2,8–11 However, thermal sintering in ambient environment cannot be used, because Cu NP is easily oxidized.² So, an inert environment have been used to sinter the ink,^8,12,13 which will increase the processing costs.⁸ To alleviate this issue of Cu ink, plasma sintering was proposed, but the method is a rather slow process.¹⁴ Recently, photonic sintering methods, such as intense pulsed lighting (IPL) and laser sintering, are proposed^2,14,15 for the sintering of metal-based NP ink. The IPL scalable xenon flash lamp, which emits light having a wide wavelength spectrum, was used to irradiate relative large areas at a time (usually at cm² scale).¹⁴ But during the sintering process, some un-wanted part could be affected and damaged. In some applications, such as broken circuit repair or localized patterning applications, selective sintering is preferred, rather than area-based sintering. Indeed, laser sintering has been effectively used in localized sintering applications, because it has the advantages of high-resolution and high-speed scanning capability.^12,14,16 The photonic sintering approaches, including IPL and laser sintering methods, have the advantage of solving the oxidation issue of Cu NP ink. For example, CuO NP ink can be used to obtain Cu conductive patterning by reducing CuO. Recently, C. Paquet et al. and M.S. Ragers et al. both reported the use of IPL in order to reduce CuO to Cu and fuse Cu particles to obtain connected conductive patterns.^17,18 However, to the best of our knowledge, the selective sintering of CuO NP ink by using green laser has not been reported in the literature. The use of green laser has the advantages of less affecting of transparent substrates, such as glass, PET films, etc., while heat is mainly generated on the irradiated printed patterns.

In this study, we investigated the sintering effects by varying laser parameters to obtain proper sintering results. It has been well known that a soft-baking (or pre-baking) process is required prior to photonic sintering of NP ink to dry out the solvent in it. However, the soft-baking effects on the sintering results have not been discussed in the literature. It has been believed that the solvent in the film should be dried out sufficiently prior to photonic sintering. However, over-baking could adversely affect the sintering results, because the deposited CuO layer could be easily damaged via laser irradiation. In this paper, we discuss the pre-baking time or temperature effects on the sintering. One of the most difficult issues for photonic sintering is to sinter thick film with more than 500 nm thickness, since most of the light is absorbed within dozens of nanometers in depth, where the heat is generated. The rest of the film (the deeper thickness) should be sintered via heat transfer from the surface. In this work, two different thicknesses of CuO films of (546 and 889) nm are used to understand the thickness effects on the laser sintering results. After sintering, the thickness of the film is significantly reduced, due to the burning of organic additives and fusion of Cu particles. We discuss the thickness reduction behavior in relation to the resistivity of the sintered Cu line. Finally, we discuss the optimal range of laser power and scanning speed to obtain proper conductive Cu line by sintering CuO NP film.

For the experiment, we used commercially available CuO NP ink (Novele IJ-220, Novacentrix, USA), which contains 16 wt% of CuO, and the particle size is about (100 –130) nm. The main solvents of the ink are water and ethylene glycol. The viscosity of the ink is ∼ (9–12) cPs, which is suitable for inkjet printing. For sintering experiment, CuO NP ink was spin-coated onto a glass substrate (Microscope slides, Marienfeld Superior, Germany) with two different speeds of (1,000 and 2,000) rpm) for 30 s, to obtain the CuO films with two different CuO thicknesses. Then, the coated ink on the substrate was soft-baked at different temperature conditions of (80, 150, and 200) °C for (20, 30, 40, and 60) min via hot plate in ambient atmosphere, to dry out the solvent, prior to laser sintering. After completing the drying process, CuO films were sintered using a focused green laser with wavelength of 532 nm (Class iv laser, Laserglow Technologies, Canada), which has a spot size of 80 μm in diameter. Figure 1 shows the experimental setup for laser sintering.

To find the optimal laser sintering parameters, the laser power was varied from (0.1 to 3.0) W with different scanning speed ranging (1 to 500) mm/s. The laser light is irradiated on the spin coated CuO film on the glass substrate after soft-baking, to form conductive lines with 2 mm line spacing. After completing the sintering, ultra violet (UV) pulsed laser with wavelength of 355 nm (Pulse 355-5, SuZhou Inngu laser, China) was used to ablate the un-sintered parts between sintered lines to separate the sintered lines, as shown in Fig. 2. The ablation has two purposes: 1) To measure the thickness of the sintered and non-sintered part using a 3D profiler; 2) to electrically separate the sintered lines, to avoid possible interactions among them. Then, the resistance of the Cu line was measured by two-point probe. The distance of two probes, L, is about 500 μm and the resistance R is measured by multi-meter (TK-4002, Chekman, South Korea). Then the resistivity (ρ) can be calculated using Eq. (1):

where, R and A are the measured resistance and cross-sectional area of the sintered line, respectively. Here, the cross-sectional area is calculated from the line width multiplied by the sintered thickness. The sintered thickness is measured by 3D profiler (ET200, Kosaka Laboratory Ltd. Japan), as well as cross-sectional microstructure image taken by using Focused Ion Beam (FIB, Lyra 3, Tescan, Czech Republic). To understand the surface microstructure after sintering, Scanning Electron Microscopy (SEM) image was taken by high-resolution scanning electron microscopy (HRSEM) (Mira 2, Tescan, Czech Republic). Also, microscopy (XTCam-D310M, Mitutoyo Measuring Microscope, Japan) was used to observe the optical microscopic image to understand the sintered line width and overall sintered status. Figure 2 shows the schematics of the experimental procedure.

Since drying and laser sintering are based on the thermal process, it is important to understand the thermal behavior of CuO NP ink. For this purpose, we measured the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of CuO NP ink, as shown in Fig. 3. For the measurement, the ink sample was heated up from (30 to 800) °C at a ramp rate of 10°C/min under nitrogen atmosphere for both TGA and DSC analysis. Figure 3 shows that the rate of weight loss started to increase rapidly around 50 °C, which showed good consistency with the DSC results. The DSC result shows strong endothermic response at 80 °C, where one of the solvents in CuO NP ink evaporated, showing good correlation with Ref. 19. From TGA analysis, we observed that most of the solvents were evaporated at 200 °C, and the organic additive added to the ink was completely decomposed within 400 °C. The total loss of weight was about 84 %, which corresponds to the weight percentage of CuO NP of 16 wt%.

For sintering experiment, the CuO NP ink was spin-coated at 2,000 rpm for 30 s on a glass substrate. In this study, the spin coating method is used for CuO NP ink deposition on the substrate, because it can easily control the thickness of CuO film layer on the substrate. Prior to sintering, it is important to dry out solvents that remain in the coated CuO film, in order to achieve good sintering results. The TGA data shown in Fig. 3 show that the temperature for soft-baking should be more than 80 °C.

Based on TGA analysis, spin-coated CuO film was pre-baked on the hot plate at 80 °C for 30 min to dry the remained solvents in the film. After the soft-baking, the thickness of the dried CuO film was measured by 3D profiler, and the measured thickness was about 520 nm.

After pre-baking, laser scanning experiment was performed with various power and scanning speeds. After completing the sintering, the resistance of the sintered line was measured via two-point probe with a probe distance of 500 μm. Then, the line width and thickness of the sintered Cu pattern were measured by optical microscopy and 3D profiler respectively. By using Eq. (1), the resistivity was calculated and plotted as a function of laser power and scanning speed as shown in Figs. 4 and 5. Figure 4 indicates that to obtain a Cu line with low resistivity, the laser power should be (0.3 and 0.5) W. The low resistivity of the sintered line means that CuO is reduced to Cu during photonic sintering, and the Cu particles are fused to form conductive lines. When the laser power was less than 0.1 W, the resistivity was too high or infinite. On the other hand, when the power became more than 1.0 W, the irradiated line was damaged with infinite value of resistivity.

Figure 5 shows the scanning speed effect on the resistivity of a sintered CuO ink when using laser power of (0.3 and 0.5) W. As shown in Fig. 5, if the scanning speed was less than 10 mm/s, the sintered line was not conductive. The scanning speed less than 10 mm/s could damage the irradiated part, due to too much laser energy on the film. In the case of higher speed greater than 100 mm/s, a conductive line could not be obtained, since the laser energy could not be converted to heat during the short time of irradiation. Figure 5 clearly shows that in the case of using laser power of 0.3 and 0.5 W, the scanning speed should be in the range of (10 to 50) mm/s to obtain the conductivity. Figures 4 and 5 show that when the laser power of 0.5 W with scanning speed of 30 mm/s was used for sintering, the resistivity of 15.7 μΩ·cm could be obtained.

To demonstrate Cu line patterning capability, green laser was irradiated on coated CuO film along the target patterns by moving XY stages. In order to make the CuO film on a glass slide, CuO ink was spin-coated at 2,000 rpm for 30 s on it. The coated film was pre-baked for 30 min on hotplate at 80 °C. Then, laser light with power of 0.5 W and scanning speed at 30 mm/s was used for the Cu line patterning. Fig. 6(a) shows an example of patterned Cu lines with size of 10 mm × 14 mm. Note that the laser irradiated patterns became brighter than non-irradiated part, which demonstrate the capability of Cu line patterning via laser irradiation. Microscopic images in Figs. 6(b) and (c) indicating that the patterned line width was about 70 μm.

Since CuO NPs is non-conductive in nature, CuO NPs should be reduced and sintered to form conductive Cu lines. The mechanism of reduction of CuO NPs has been discussed in literatures.^18,20 When the intense light is irradiated on the deposited film, the CuO NPs are heated instantly and thereby the temperature of agents and solvents increases to react with the copper oxide and convert it to copper metal. The temperature spike is so brief that the newly reduced copper does not have time to oxidize so that the entire process can be conducted in air. For reduction and sintering of the CuO ink, the use of intensive pulse light (IPL) has been recommended. Since we are using green laser instead of intensive pulse laser for the sintering, we verified the reduction of CuO to Cu via crystal phase analysis based on x-ray diffraction (XRD) (Rigaku, Miniflex600, Japan) as shown in Fig. 7. For this purpose, two samples of sintered and non-sintered cases were prepared to compare the XRD patterns. For the non-sintered sample, CuO NP ink was spin coated on the glass substrate with 2,000 rpm for 30 s, and soft-baked for 30 min on hotplate at 80 °C. After pre-baking, the CuO coated glass was cut so that the size of the sample could be 1.0 cm × 1.0 cm, in order to fit it into the sample holder of the XRD equipment. The other sample was prepared such that the pre-baked CuO film was irradiated by green laser with power of 0.5 W and scanning speed of 30 mm/s to obtain a sintered area of size 1.0 cm × 1.0 cm. Here, the sintered Cu line width was about 70 μm, and the spacing between irradiated lines was about 50 μm, so that the irradiated lines could be slightly overlapped.

The non-sintered CuO NP film in Fig. 7(a) shows that peaks in the XRD pattern were observed at (35.5, 38.7, 48.6)° etc., which matches the CuO pattern. On the other hand, Fig. 7(b) shows the XRD pattern of sintered CuO film, which shows Cu crystal phase with peaks at (43.3, 50.43, and 74.13)°. This XRD observation indicates that green laser sintering was effective to reduce CuO to Cu.

To understand the sintering behavior according to various sintering conditions, we investigated the surface morphology based on SEM images. Figure 8 shows the SEM images of the surface of the CuO film before and after laser irradiation. Figure 8(a) shows that the surface of the non-sintered SEM image after soft-baking indicates that the grain structure of NP is not fused, but the particles are distributed uniformly on the surface. Figure 8(b) shows the surface morphology of the incompletely sintered case due to low laser irradiation with laser power of 0.1 W and scanning speed of 100 mm/s. In both cases, we obtained infinite resistivity. When we increased the laser power to 0.5 W and lowered the scanning speed to (30 and 50) mm/s, the particles became connected, and grain growth could be observed, as shown in Figs. 8(c) and (d). As a result of particle connections, the resistivity ranging from (15 to 20) μΩ·cm could be measured from the sintered lines.

On the other hand, in the case of using high power of (0.5 and 1.0) W with low scanning speed of (1 and 10) mm/s, the coated CuO NP layer was totally damaged, as shown in Figs. 9(a) and (b). When excessive laser energy was irradiated, the sintered particles became irregular, and the pattern was totally destroyed by merging one another into separated micrometer-sized Cu particles. Figure 9(c) shows that if the scanning speed becomes as low as 5 mm/s, not only the Cu line part, but also the substrate could be damaged. By applying excessive laser power with low scanning speed, the temperature of CuO NP ink layer can increase significantly due to plasmonic resonance, since organic molecules can absorb high energy below 700 nm laser source (for higher electronic excitation).²¹ After absorption of laser light, we observed from XRD analysis that CuO was converted to Cu, as shown in Fig. 7. The reduced Cu particles became sintered to form a connected Cu line. The converted Cu line has a high absorption rate in the green laser wavelength of from (500 to 800) nm.¹⁶ As a result, heat can be generated further up to 1,500 °C.²² In the case of using low scanning speed, we often observed breakage of the glass, due to the abrupt increase in temperature. On the other hand, in the case of high power of 3.0 W with high scanning speed of 100 mm/s, the glass is less damaged, as shown in Fig. 9(d), because CuO film may not be able to absorb sufficient laser energy, due to the short laser exposure time, in spite of the high laser power.

Note that oxygen can be generated inside film during the reduction process of CuO. As a result, there can be voids in the sintered line, as shown in Figs. 8(c) and (d). The voids in the sintered line could be one of the primary causes of low resistivity.^13,23 In the case of voids trapped inside the film, it may be difficult to understand the sintering quality by investigating the surface morphology based on the SEM images only. To understand the degree of voids in the sintered line, the thickness of sintered line was measured by surface profiler. In the case of using different scanning speed of (30 and 50) mm/s with 0.5 W laser source, the thicknesses were measured to be (148 and 169) nm, respectively as shown in Fig. 10. Compared to the non-sintered thickness of 520 nm, the thickness of sintered lines was significantly reduced up to 72 % (from (520 to 148) nm). Then, the resistivity was measured to be 15.7 μΩ·cm, which is 9 times that of bulk Cu, and similar to the previous studies of CuO NP sintering via IPL.¹⁸ It is clear from our results that the thickness reduction is clearly related to the conductivity of the sintered lines. Alternatively, we also used FIB analysis to understand the voids behavior inside of the sintered lines, which will be discussed later.

To understand the soft-baking effects on sintering, three samples were prepared with different soft-baking times of (20, 30, and 40) min on 80 °C hotplate. Note that the initial thicknesses prior to laser sintering were measured to be about 546 nm (measured by FIB), irrespective of the drying time. Then, laser sintering was performed for each sample with laser power of 0.3 W and scanning speeds of (10, 20, and 30) mm/s. Figure 11 shows the sintering results according to the soft-baking times. The figure shows that the drying time significantly affected the resistivity, and there can be an optimal condition for soft-baking time. In our experiment, we obtained the resistivity of 9.5 μΩ·cm when using 30 min soft-baking, which is similar to the previous resistivity of the IPL/flash white light sintering results of (9 and 10) μΩ·cm in Refs. 17 and 18.

For better understanding of the soft-baking effects, we compared the SEM images of laser sintered CuO film layer as shown in Figs. 12(a), (c), and (e), where soft-baking conditions were 80 °C with (20, 30, and 40) min, respectively. These figures show that the grain size of each result became larger, compared to the non-sintered cases shown in Figs. 8(a) and (b). On the other hand, we can also observe some micro-voids, which are the primary causes of low resistivity. However, it is not very clear from the surface morphology whether the sintering conditions were optimal, because we could not understand the microstructure inside the sintered line. For better understanding, we have investigated the cross-sectional micro-structures via FIB, as shown in Figs. 12(b), (d), and (f). Note that Platinum (Pt) coating was required to protect the Cu layer while cutting the film using gamma rays. The FIB images show that the sintered thickness was reduced according to the increase of soft-baking time. The thickness reduction might mean less voids in the sintered lines. Figures 12(b), (d), and (f) show the reduction of voids as the soft-baking time increases. When the soft baking times were (20, 30, and 40) min, the sintered thicknesses were (254, 141, and 130) nm, respectively. Note that the original thickness prior to sintering was 546 nm. The thickness reduction is related to converted Cu film compaction with pore annihilation. As the drying time increases, the particles can easily fuse with one another, so that the layer becomes dense. However, we note that the resistivity may not be directly related to the thickness. For example, in the case of soft-baking of 40 min, the thickness becomes 130 nm, which is thinner than the 141 nm of 30 min baking. But, the resistivity was higher at about 21.8 μΩ·cm, compared to the 9.5 μΩ·cm of 30 min soft-baking time. Nevertheless, in general, thickness reduction is a good evaluation method for the sintering condition. It has been believed that solvent should be fully evaporated prior to sintering. However, from the experiment, there are optimal conditions for pre-baking. The soft-baking effects on resistivity are investigated further in the next section, when we discuss the sintering of thicker CuO film.

In this section, we investigate the sintering behavior of thicker CuO film. If the CuO film becomes thicker than 500 nm, it is difficult to obtain optimal conditions for uniform sintering throughout the thickness direction. To prepare a sample with thicker CuO film, CuO NP ink was spin-coated on a glass substrate with low rotating speed of 1,000 rpm for 30 s. With the increase in thickness, the time and temperature for soft-baking conditions need to be increased accordingly to dry the solvent in it. For the soft-baking, three different temperatures of (80, 150, and 200) °C were used to prepare three test samples. The TGA analysis shown in Fig. 3 shows that to evaporate all the solvents, the temperature needs to be more than 150 °C. After baking for 1 h, the thickness of each sample was measured to be about 889 nm, irrespective of the soft-baking temperature. Then, green laser of 0.5W was irradiated on the baked film to obtain sintered line with different scanning speeds from (1 to 500) mm/s.

Figure 13 shows the optical microscopy images of sintered CuO film according to soft-baking temperatures and laser scanning speeds. The figure shows that if the soft-baking temperature increases to more than 80 °C, the conductive lines cannot be obtained. Note that when the baking temperature is 80 °C, some solvents are likely to be remained. However, in the case of using temperature higher than 150 °C, almost all solvent remained in the film was likely to be evaporated. In such case, the laser irradiation could significantly increase the temperature on the film, and the heat was easily transferred in the thickness direction, resulting in damage to the film. Based on our experimental results, some solvent remaining in the ink might help to avoid damage in the sintered line. The previous results in Fig. 11 show similar behavior, indicating that there is an optimal time for pre-baking. The role of slight remaining solvents became critical in the case of green laser sintering of thick CuO film more than 500 nm, since more heat is required, and thus the generated heat can easily damage the film.

To further investigate the resistivity of sintered line, we used CuO films prepared by soft-baking at 80 °C for 1 h. After pre-baking, the thickness of the film was measured to be 889 nm. For the sintering, we used different laser powers of (0.1, 0.3, 0.5, and 1.0) W. Following the similar procedure in Section III B 2, the resistivity of Cu line was calculated and plotted with respect to scanning speed, as shown in Fig. 14. The figure shows that to obtain conductive lines, the laser power should be in the range (0.1 to 1.0) W. The resistivity of 71.6 μΩ·cm could be obtained when the laser power of 0.5 W and scanning speed of 50 mm/s were used. However, the resistivity was relatively high at about 71.6 μΩ·cm, which is about 42 times that of bulk Cu. As the thickness of CuO film increases, the selection of optimal parameters in order to obtain good conductivity became more difficult to find.

To understand the micro-to nano-structure of the sintered line, we measured the surface (SEM images), as well as the cross-sectional (FIB images) morphology, as shown in Fig. 15. Figure 15(a) shows the SEM image using the sintering condition of power of 0.1 W and scanning speed 10 mm/s. The figure shows no significant connection of particles. On the other hand, when using the different sintering conditions of 0.5 W and 50 mm/s, the particle connection seems to be improved, as shown in Fig. 15(c). However, the sintering behavior cannot be clearly understood by observing the surface morphology only.

To understand the sintered condition more clearly, cross-sectional FIB images were used, as shown in Figs. 15(b) and (d). At low power 0.1 W and scanning speed 10 mm/s, the thickness reduction from (889 to 594) nm was observed, as shown in Fig. 15(b). In this case, the film shows infinite resistance. On the other hand, when the sintering condition using laser power of 0.5 W and scanning speed 50 mm/s was used, the reduction from (889 to 270) nm was observed, as shown in Fig. 15(d). The resistivity was measured to be as high as 71.6 μΩ·cm, compared to the 9.5 μΩ·cm obtained for 546 nm thickness CuO layer sintering, explained in Section III B 6. By comparing the thickness reduction and observing the micro-structure in the thickness direction using FIB images, the sintering behavior with respect to sintering parameters became obvious, as shown in Fig. 15.

It was difficult to find the optimal conditions for sintering when the thickness increases, because heat is generated on the surface only, and proper heat transfer is required to obtain uniform sintered results in the thickness direction, without damaging the film. As a result, soft baking conditions, such as temperature and time, as well as the laser sintering parameters, become critical to obtain a good conductive Cu line.

In this study, we provided a general guideline for selecting sintering parameters using green laser, in order to sinter commercialized CuO NP ink. However, we believe that further research is required to understand the green laser method for a specific application using CuO ink. For example, we used a glass slide as a substrate. The sintering performance could be different according to substrate, due to different heat transfer characteristics. This study considered the use of spin-coating, due to the easy control of thickness by adjusting the rotational speed. As a result, the printed line width effects have not been considered. In practice, the printed line width might be smaller than the laser spot size, and might need consideration in practical applications.

Green laser parameter effects on the sintering of commercialized CuO NP ink were investigated in terms of laser power, scanning speed, pre-baking conditions, film thickness effects, etc. Prior to laser sintering process, it is necessary to dry out the solvents in the deposited CuO film, in order to avoid air bubbles explosion due to the sudden boiling of solvents inside of the deposited film during laser irradiation. In this study, we found that the use of proper pre-baking conditions was important to ensure good sintering results, because not only could it prevent air bubble explosion inside the film, it also enhanced the conductivity. Our experimental results indicated that slight solvent remaining in the CuO film would be needed to obtain uniform sintering with less damage of the film during the laser sintering process.

In this study, the best sintered results were obtained when laser power ranging from 0.3 to 0.5 W was used. We also found that to obtain conductivity, the laser scanning speed should not be faster than 100 mm/s, and to avoid damage of the laser irradiated parts, should not be slower than 10 mm/s. As a result of sintering, the thickness was reduced by as much as 74 % (e.g., (546 to 141) nm). Thus, the measured resistivity ranged from 9.5 to 20 μΩ·cm, which is equivalent to that of using IPL/flash white light. The higher thickness reduction meant lower resistivity, since the connectivity of nano-materials, as well as the compactness, is related to the thickness reduction after sintering.

This research was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea, funded by the Ministry of Education (2016R1D1A1B01006801). It was also partially supported by the Soonchunhyang University Research Fund.