Low temperature sintering process of copper ﬁ ne particles under nitrogen gas ﬂ ow with Cu 2+ alkanolamine metallacycle compounds for electrically conductive layer formation †

A novel low cost sintering process of copper ﬁ ne particles to a copper conductive layer was demonstrated at as low as 100 (cid:1) C without reductive gas ﬂ ow. Sintering of a mixture of copper particles and copper-based metal – organic-decomposition (MOD) ink gave a copper ﬁ lm with high packing density and low resistivity (9 (cid:3) 10 (cid:4) 6 U m). This novel process may open a new strategy in the ﬁ eld of printed electronics.


Introduction
The unique properties and applications of metal nanoparticles and ne particles have driven the development of recent materials. [1][2][3][4][5] Many efforts have been made to develop lowtemperature sintering processes for printed electronics, especially for organic devices as well as printing on conventional polymer lms. Printable processes using nanomaterials are expected to open a door for novel manufacturing processes which dramatically decreases the energy consumption compared to conventional processes. Metallic silver inks and lms with high stability and a high conductivity are one of the most representative materials of the printed electronics technology. However, silver ion migration is an old problem which still needs to be solved. Metallic copper nanoparticles and ne particles are considered as a preferable candidate according to their low cost, high conductivity, and lower electromigration. [5][6][7][8][9][10][11] However, their easy oxidation has prevented their practical use, especially for the low temperature sintering process.
In order to overcome the problems of copper, Yabuki 12 and our group [13][14][15][16] have recently studied low temperature sintering processes using a two-step strategy, i.e. oxidative annealing and reductive annealing. This two-step sintering process gave a very low sintering temperature to submicron copper ne particles. At present, the sintering temperature is still a little high (150 C, 15 200 C 16,17 or more than 200 C 13,14,18 ). We used a proton-initiating decomposable polymer as the stabilizing reagent for copper ne particles and they were sintered at 150 C. 19 In order to apply copper inks or pastes to organic electronic devices, very low sintering temperatures are demanded. For example, laser sintering is one possibility for low temperature sintering. 20,21 Recently, metal-organic-decomposition (MOD) ink has attracted considerable attention for low temperature sintering. The MOD ink of copper consists of copper salts, ligands, and some additives; this ink will decompose to form a copper lm aer sintering. [22][23][24][25][26] Kim et al. 26 demonstrated that the ultimate electrical resistivity of a lm depends on the copper concentration in a copper(II) formate (CuF)-hexylamine complex which controls the porosity and impurity content of the lm. The lowest electrical resistivity, obtained from the ink (mole ratio of Cu to hexylamine was 1 : 2.6) annealed at 200 C followed by formic acid reduction at 250 C, was 5.2 Â 10 À8 U m. Yabuki and coworkers used complexes of copper formate and various amines, and studied the relationship between the size of nanoparticles and the types of amines as well as the length of the alkyl chain. The lowest resistivity they could achieve was 5 Â 10 À8 U m at 140 C. 25 Shin et al. reported a self-reducible and alcohol-soluble copper-based MOD ink. They used CuF as the precursor, 2-amino-2-methyl-1-propanol as the ligand, octylamine as the co-complexing agent, and hexanoic acid as the sintering helper to obtain a resistivity of 2.3 Â 10 À7 U m aer sintering at 200 C under N 2 . 22 Farraj et al. investigated the mechanism of the decomposition process of the complex between copper formate and 2-amino-2-methyl-1-propanol. The lowest resistivity aer sintering under N 2 was 1.1 Â 10 À7 U m at 190 C. 23 Choi et al. elucidated the nucleation and growth behavior of copper nanoparticles which determined microstructure of lm during sintering. Using copper(II) formate as the copper source and hexylamine as the ligand, they achieved the lowest resistivity of the copper lm (4.1 Â 10 À7 U m) at 200 C under a formic acid atmosphere. 24 In the above reports, the sintering temperature (130-250 C) to achieve a low resistivity in the range of 10 À6 to 10 À8 U m is still high from a cost perspective.
In this study, we would like to propose a novel method to obtain a high conductivity copper lm at low sintering temperature based on the addition of copper particles to a complex of copper(II) formate and 1-amino-2-propanol (isopropanolamine) to form the copper ink. We expect a higher packing density of the copper lm aer decomposition of copper complex followed by the co-sintering of the added copper and generated copper particles, leading to a higher conductivity at a lower sintering temperature. Our results demonstrated that without using any organic additives and helpers as well as reductive gas during sintering, we successfully obtained copper lms with resistivity in the order of magnitude of 10 À6 U m at 100 C. To the best of our knowledge, this is the rst time that ink is prepared by mixing copper particles with copper-based MOD ink and that such low resistivity is obtained at 100 C by using a copper-based MOD ink.

Preparation of copper(II) formate-alkanolamine (CuFalkanolamine) inks
We have selected the aforementioned alkanolamines as ligands because stable, 5-membered metallacycles can be formed between the alkanolamines and copper. 10 mmol of CuF (2.26 g) was added to 10 mmol of the alkanolamines (1 : 1; mol CuF : mol alkanolamine), IPA (0.75 g), 2AB (0.89 g), and DEAE (1.17 g). These alkanolamines are bidentate. The chemical structures of the obtained CuF-alkanolamine complexes are displayed in Fig. 1. Then, the mixture was stirred to obtain homogeneous liquids. Aer CuF completely dissolved in the alkanolamines, the copper complexes were used as the MOD inks to prepare the copper lms.
Preparation of copper(II) formate-IPA-copper particle (CuF-IPA-Cu) inks CuF-IPA complexes were prepared using CuF (10 mmol, 2.26 g) and IPA (10 mmol, 0.75 g). Copper particles with median diameters of 0.8 mm (Fig. S1 †) were added into the as-prepared Cu-IPA complexes using a molar ratio of 1 to 6 (CuF to copper particle), followed by mixing using a conditioning mixer less than 30 min to form the CuF-IPA-Cu inks. The total concentration of Cu in the ink was approximately 65 wt%.

Preparation of copper lms
The as-prepared CuF-alkanolamine inks and CuF-IPA-Cu inks were deposited on alumina substrates using a doctor blade (used thicknesses of the printed areas were 100 mm and 40 mm, respectively) and sintered in a tube furnace at 100 C under N 2 gas at a ow rate of 1 dm 3 min À1 . 99.99% N 2 gas from a cylinder was used for sintering the CuF-alkanolamine inks, and 99.9% N 2 gas generated using a nitrogen supplier (model 05, System Instruments, Japan) was used for sintering the CuF-IPA-Cu ink. The obtained copper layers were characterized and used for further measurements.

Characterizations and measurements
Thermogravimetric-differential thermal analysis (TGA-DTA) measurements were performed using a Shimadzu DTG-60H. X-ray diffraction (XRD) patterns were obtained using a Rigaku Miniex-II diffractometer. Scanning electron microscopy (SEM) images of the copper particles and copper lms were collected using a JEOL JSM-6701F eld-emission type SEM. The resistivity of copper lms was measured by a four-point probe method using a Mitsubishi Chemical Analytech Loresta-GP with an ASP probe.

Metal source and alkanolamines
The most important advantage of CuF is its self-reduction ability, as it forms copper and releases hydrogen gas during the decomposition. 27,28 Moreover, alkanolamines also show reduction abilities. 9 This facilitates the reduction of Cu 2+ from CuF to Cu(0), and prevents the oxidation of copper during sintering and reduces surface oxides from the commercially available ne copper particles. The low decomposition temperature of the CuF-alkanolamine complexes allows for the formation and sintering of copper particles to be performed under nitrogen gas only, thus negating the need for the use of reductive gases such as hydrogen or formic acid. 24,26 At the same time, the low decomposition temperature meets the requirement of low temperature and cost for printed electronics, that is, printable on the plastic substrates with low glass transition temperature as well as on organic electronics parts. Finally, the decomposition of CuF results in H 2 and CO 2 , without leaving any organic residues that can decrease the conductivity of the lm. Usually, metallacycle complexes exhibit high stability. These complexes also exhibit low evaporation or decomposition temperatures.

Stability of copper(II) formate-alkanolamine complexes
Alkanolamines including IPA, 2AB, and DEAE were selected to study the stability of MOD inks prepared using a 1 : 1 molar ratio of CuF and the alkanolamine. Fig. 2 shows the colour changes of different CuF-alkanolamine complexes over time. For the CuF-IPA and CuF-2AB complexes, there were no obvious colour changes aer 14 days and 6 h, respectively. However, some solid components formed with the Cu-2AB complex aer 4 h, as shown in Fig. 2b. In contrast, the colour of the CuF-DEAE complex began to change from indigo to light blue aer 1 h (Fig. 2c). The colour of the CuF-DEAE complex continuously changed to light turbid blue with time. This indicates the formation of solid materials with light blue colour in the liquid.
XRD measurements of the CuF-alkanolamine complexes (Fig. 3) were performed. Aer preparation, all the CuF-alkanolamine complexes exhibited an amorphous phase as noted by the XRD patterns. With a storage time of up to 14 days, negligible changes were observed in the XRD pattern of the CuF-IPA complex. The XRD patterns of the CuF-2AB complex showed intense peaks aer keeping for 4 h, indicating the formation of crystalline products. Unfortunately, the solid products could not be identied. On the other hand, aer only 0.5 h, the XRD pattern of the CuF-DEAE complex revealed peaks related to crystalline phases. By comparing this pattern with JCPDS cards, we found that copper formate hydroxide (CH 2 CuO 3 , Cu(H-COO)(OH)) was the main component of the corresponding solid (Fig. S2 †). 29 This compound is greenish blue, and the colour change observed in Fig. 2c can be explained by this compound.
In both cases, the XRD peaks became sharper and more dened with increased storage times. Based on these results, it was concluded that among the alkanolamines used in this study, IPA formed the most stable complex with CuF.
The aforementioned results were attributed to the stability of the structures of different CuF-alkanolamine complexes (Fig. 1). According to hard so acid base theory, 30 IPA and 2AB can form bidentate interactions with copper, resulting in vemembered metallacycles with stable structures. However, as DEAE is a tertiary amine with two ethyl groups that bind to N, it is sterically hindered. Therefore, the bidentate structure shown in Fig. 1c should be more unstable than those of the other two complexes. Therefore, Cu(HCOO)(OH) was generated and formed a solid powder.
IPA and 2AB have similar molecular structures ( Fig. 1a and  b). However, CuF-IPA did not form any solid compounds, probably due to differences in the self-assembly of the two complexes. CuF-2AB complexes may stack on top of each other owing to their amphiphilic structure. Therefore, no colour change was observed in Fig. 2b, but self-assembled solid components were formed aer several hours. We chose IPA as the optimal ligand for complex formation with CuF in the next step of our study due to the best stability of the CuF-IPA complex.

Conductivity of copper lm prepared from CuF-IPA complex
In order to investigate the resistivity of the copper lm fabricated using the CuF-IPA complex, the printed complex was sintered under N 2 at 100 C. Layers of CuF-IPA complex were prepared using a doctor blade with a thickness of 100 mm.
The blue layer of the CuF-IPA complex turned into a brown lm ( Fig. 4a and b) aer 1 h of sintering at 100 C under nitrogen ow (99.99%). In the XRD pattern of the sintered  sample, all of the observed peaks belonged to metallic copper. This XRD pattern clearly indicates that reduction of Cu 2+ to Cu(0) proceeds at 100 C in the presence of IPA. The formation of metallic copper was due to the decomposition of CuF accompanied by the generation of H 2 reductive gas. Aer reduction of Cu 2+ , IPA evaporates slowly because the boiling point of IPA is 160 C. The decomposition of the CuF-IPA complex was also conrmed using TGA-DTA (Fig. S3 †). Decomposition of CuF-IPA occurs even at temperatures lower than 100 C. It is possible that copper ions were also reduced during heat treatment of CuF-IPA. 22 However, when only the CuF-IPA complex was used for printing, it was difficult to obtain a uniform copper lm aer sintering ( Fig. 4a-b). Conductivity was not detected for the copper lm obtained aer sintering the CuF-IPA ink at 100 C, owing to the formation of many cracks and the inhomogeneity of the copper lm. This problem was also reported by Shin et al. when 2-amino-2-methyl-1-propanol was used as the ligand and isopropyl alcohol was used as the solvent to prepare a copper complex ink. 22 Copper lms prepared from CuF-IPA-Cu inks To overcome the aforementioned problems, instead of using organic additives or sintering helpers as in previous studies, [22][23][24][25][26] we introduced copper particles into the CuF-IPA complex to make the ink. The total copper concentration in this ink is approximately 65 wt%. In this way, the use of organic additives and sintering helpers as well as a reducing gas could be avoided while the packing density of the copper lm was improved aer the decomposition of the copper complex followed by the cosintering of the added copper and generated copper particles. This can lead to a higher conductivity of the resulting copper lm at a lower sintering temperature.
The sintering temperature was reduced to 100 C and the resistivity of the lm was measured using the sample prepared by adding 0.8 mm-copper particles. A particle size of 0.8 mm was optimal in this study because in smaller particles, many protective agents, which cover the copper particles, prevent interactions between the copper particles during sintering. Conversely, bigger particles lead to a poor packing density owing to the large pore volume. Fig. 5c shows the SEM image of the sintered copper lms obtained from the CuF-IPA-Cu ink. The connections among copper particles were observed in the lms prepared using 0.8 mm-copper particles aer sintering for 1 h at 100 C. Under these conditions, a resistivity of 9.0 Â 10 À6 U m was obtained. The relatively lower concentration of IPA contributed to the higher packing density of the obtained copper lm. Fig. 5a and b show the photographs of the CuF-IPA-Cu ink aer printing at room temperature and aer sintering at 100 C, respectively. Compared to the sintered lm from the CuF-IPA complex, the sintered copper lm prepared from the CuF-IPA-Cu ink exhibited much higher uniformity. No cracks were observed on the surface of the lm. By adding copper particles into the ink, the packing density was signicantly improved aer sintering at 100 C. Tight connections among copper particles were observed in the SEM image (Fig. 5c) of the lm aer sintering, Fig. 4 (a-b) Photographs of CuF-IPA complex prepared using CuF and IPA (1 : 1 molar ratio) before and after sintering at 100 C under N 2 for 1 h, respectively; (c) XRD pattern of the film shown in (b). Fig. 5 (a-b) Photographs of CuF-IPA-Cu ink after printing and after sintering at 100 C under N 2 for 1 h, respectively. (c) Cross sectional SEM image of the copper film shown in (b). The CuF-IPA-Cu ink was prepared using 0.8 mm copper particles and a 1 : 1 : 6 molar ratio of CuF to IPA to copper particles. which resulted in the low resistivity. This result demonstrated the effectiveness of the developed method in terms of producing a lm with high conductivity at a low sintering temperature.

Conclusions
In conclusion, highly conductive copper lms were achieved at a very low sintering temperature of 100 C by using a copperbased MOD ink and without reducing gas for the rst time. The avoidance of reducing gases, such as hydrogen, makes the system safer and simpler. The addition of copper particles to CuF-IPA complexes at a sintering temperature of 100 C resulted in a lm with a resistivity of 9.0 Â 10 À6 U m. The developed sintering process had signicant advantages in the production of a highly conductive lm for printing, including: (i) enhanced packing density, smoothness and uniformity of the copper lm based on the direct sintering of the added copper particles and copper particles generated during the decomposition of the copper complex, (ii) avoided the use of organic additives and sintering helpers as well as reducing gas, and (iii) reduced energy and time. Sintering at 100 C enabled us to use this system with PET lms. Optimization of detailed sintering parameters and preparation of stable pastes or inks are underway in our laboratory and will be reported in due course.