Reverse offset printing of semi-dried metal ... - ACS Publications

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Letter

Reverse offset printing of semi-dried metal acetylacetonate layers and its application to a solution-processed IGZO TFT fabrication Yasuyuki Kusaka, Naoki Shirakawa, Shintaro Ogura, Jaakko Henrik Leppäniemi, Asko Sneck, Ari Alastalo, Hirobumi Ushijima, and Nobuko Fukuda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07465 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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ACS Applied Materials & Interfaces

Reverse offset printing of semi-dried metal acetylacetonate layers and its application to a solution-processed IGZO TFT fabrication Yasuyuki Kusaka a) *, Naoki Shirakawa a), Shintaro Ogura a), Jaakko Leppäniemi b), Asko Sneck b), Ari Alastalo b), Hirobumi Ushijima a), Nobuko Fukuda a) a) Flexible Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan *E-mail: [email protected] b) VTT Technical Research Centre of Finland Ltd. Tietotie 3, Espoo FI-02150, Finland

Abstract. The sub-micrometer resolution printing of various metal acetylacetonate complex inks including Fe, V, Mn, Co, Ni, Zn, Zr, Mo and In was enabled by a robust ink formulation scheme which adopted a ternary solvent system where solubility, surface wettability and drying as well as absorption behavior on a polydimethylsiloxane sheet were optimized. Hydrogen plasma in heated conditions resulted in bombarded, resistive or conductive state depending on the temperature and the metal species. With a conductivity-bestowed layer of MoOx and a plasma-protecting layer of ZrOx situated on the top of an IGZO layer, a solution-processed TFT exhibiting an average mobility of 0.17 cm2/Vs is demonstrated.

In the emerging field of printed electronics, reverse offset printing (ROP) has received a growing attention for the fabrication of miniaturized devices because of its ability to create single micrometersized patterns with an excellent thickness uniformity1-5, which are unattainable by other conventional methods such as inkjet printing, gravure offset printing and screen printing. Examples of demonstrated devices include color filters6, organic thin-film transistors (TFT)1,7,8, inverters5, transparent electrodes2,9, memories10 and antennas. Briefly, in the ROP process, ink applied to a polydimethylsiloxane (PDMS) surface is patterned by transferring the unnecessary parts of the ink ACS Paragon Plus Environment

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layer from the PDMS sheet to a relief structure called the cliché. As an ink layer applied on the PDMS can be promptly solidified by evaporation and solvent uptake by the PDMS1, the patterning in ROP is achieved by the adhesion strength of the semi-dried ink layer against the cliché accompanied by an external shear stress applied by the edges of the cliché reliefs3. Therefore, not only wettability and film-forming capability on the hydrophobic PDMS but also the patterning ability owing to the fracture of the semi-dried layers is required for the ROP ink formulation. Thus far, the inks applicable to ROP have been limited to colloidal inks, such as silver nanoparticles and quantum dots11, and oligomeric polymer inks, such as poly(3-hexylthiophene) organic semiconductor, and dielectric polymers1. On the other hand, metal complex, salt and alkoxide precursor solutions offering a large variety of oxide-based materials12 have paved the way for the fabrication of printed electronic devices13-15 such as solution-processed metal oxide TFTs, resistive memories and sensors. However, previously reported methods such as inkjet printing16,17 and flexographic printing18,19 are limited to patterning resolutions of abut 20 µm and 50 µm, respectively, and also suffer from the variation in layer thickness and printed shape as well as from irreproducibility. Further, printing of source/drain (S/D) electrodes particularly for n-type semiconductors is a challenge as ROP-compatible printable metal inks is only limited to silver and copper at present. To overcome those problems, in this letter, we explored the potential of high-resolution ROP patterning of precursor solutions of various metal elements, particularly focusing on the metal acetylacetonate (acac) complexes. The post-annealing process of a hydrogen plasma was also examined to endow functionalities to ROP-processed layers. Finally, a fully-printed IGZO-TFT with MoOx S/D electrodes and a plasma-masking layer of ZrOx was demonstrated. For the ROP ink formulation of metal/metal-oxide precursor solutions, we primarily chose a binary mixture of toluene and methanol as methanol generally dissolves the acac complexes and toluene is easily coated on a slightly vacuum ultraviolet (VUV)-treated PDMS surface without any surfactant additives. In the following, we use MoO2(acac)2 to demonstrate the strategy for the development of ROP-compatible precursor inks. Firstly, to simplify the system, the content of MoO2(acac)2, toluene and methanol were fixed at mMo = 0.1 g, mtol = 2.0 g and mMeOH = 0.16 g, respectively. Additionally, a

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less volatile solvent of iso-pentanol mi-PenOH was added to control the solidification characteristics of the ink applied on the PDMS. As toluene and methanol are rapidly lost when compared to iso-pentanol due to their volatility and penetration into PDMS that results from their high affinity with PDMS20,21 (see Table S1 for the evaporation rate and PDMS uptake rate of the corresponding solvents), the solid content of the ink layer in the semi-dried state is represented by fsd = mMo / (mMo + mi-PenOH). In the preparation of the inks, MoO2(acac)2 complex was dissolved in the corresponding solvent, sonicated for 3 min, heated at 80 °C for 5 min and then filtered through a 0.45 µm PTFE membrane. Figure 1 shows a ROP process margin diagram of MoO2(acac)2 complex inks with a varied amount of iso-pentanol. In the ROP test, the inks were coated with a capillary slit coater (125 µm gap) at the coating speed of 30 mm/s on a 25 µm-thick PDMS surface fixed on a printing roller. We note that the PDMS surface was treated by VUV exposure (excimer light source with the wavelength of 172 nm) for 10 s with the intensity of 10 mW/cm2 to enhance the hydrophilicity. As seen in the figure, in the absence of iso-pentanol (fsd = 100 wt%), no temporal margin for good transferability after coating was found. With decreasing fsd, the printing margin gradually widened but some transfer failures began to occur with longer waiting times. In an optimal condition of 50 wt% < fsd < 60 wt%, a sufficient margin of tt > 600 s to carry the RO printing process was attained. With the further decrease of fsd, a perfect transfer was still possible; however, partial dewetting was observed. This can be attributed to the slower solidification of the applied ink layer compared with dewetting kinetics at the surface of PDMS.


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Figure 1. (a) Temporal printing margin of MoO2(acac)2 inks with varied fractions of iso-pentanol. Typical images of transferred ink layers for the cases of fsd = (b) 25, (c) 50, and (d) 100 wt%. Printing condition: 23 °C and 30 ± 5 %RH.

On the basis of the printing margin test of MoO2(acac)2, the ink formulation scheme with the ternary mixture of toluene, iso-pentanol and methanol was applied for other metal complexes in the following experiments. Slight modifications on solvent fraction were made depending on the solubility of the materials (see Table S2). We tested the compatibility of ROP pattering for the acetylacetonate or oxoacetylacetonate complexes of V (III), V (IV), Cr (III), Mn (II), Fe (III), Co (II), Co (III), Ni (II), Cu (II), Zn (II), Ga (III), Zr (IV), Mo (IV) and In (III) elements. Firstly, a preliminary screening was carried out by spincoating the aforementioned inks at 1800 rpm for 30 s on a silicon wafer followed by a pre-annealing at 130 °C for 5 min. As a result, the layers of Cr, Co (III), Cu and Ga (acac) complexes exhibited roughened appearance with microcrystals inside the thin films after the pre-annealing. Therefore, those metal complexes were omitted in the following printing test. Figure 2a shows the interdigitated patterns of the optimized metal (acac) inks by ROP (see Table S2 for the formulae) with linewidth/spacing of (L/S) = 5/1.2 µm. For all cases of the metal (acac) complexes, well-defined patterns with a rectangular cross section were generated. Remarkably, the present ink also manifested the minimum space resolution of down to 0.2 µm (Figure 2b), which enables the possibility for further miniaturization of the printed functional patterns. Those unique features, being preferable for devicefabrication perspective, can be attributed to the present semi-drying characteristics shown in Figure 1 as the layer topologies in a quasi-solid state no longer suffer from Laplace pressure, which normally affects the ink shapes deposited on workpieces22 in other printing methods including micro-contact, flexographic and inkjet printing.


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Figure 2. (a) L/S = 5/1.2 µm interdigitated patterns of various metal acetylacetonate/oxoacetylacetonate inks printed by reverse offset printing. (b) AFM topologies and height profiles of printed MoO2(acac)2 layers. Left: L/S = 5/3 µm and right: L/S =2/0.2 µm.

Among the metal complex inks examined here, printed layers of transition metal (acac) complexes of Mo (VI), V (IV) and Co (II) were found to be reducible to conductive states by hydrogen plasma treatments. Figure 3 shows the effect of the H2 plasma conditions on the resistivity of the corresponding materials (the temperature of the plasma chamber stage Tp and exposure time tp were varied while output power of 1.5 kW with 2000 sccm of helium gas with 3% hydrogen was kept constant). The test pattern for the resistivity measurements is shown in Fig. S1. In the case of molybdenum, the resistivity appreciably reached plateau values of the order of 10 mΩ cm when the samples were exposed to H2 plasma for tp > 60 s at Tp = 250 °C, or at Tp > 225 °C for tp = 120 s. For the cobalt (II) and vanadium (IV), higher resistivities in the order of 102 Ω cm and 104 Ω cm, ACS Paragon Plus Environment

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respectively, were obtained. Here, we note that the reducibility of our inks is well related to the magnitude of the equilibrium constants23 for reduction reactions as metal oxides requiring partial pressure of oxygen of above p(O2) = 10-54 atm at 200 °C (Log[p(O2)] = -54) for reduction corresponded to the metal complexes that manifested detectable resistivity after the H2 plasma treatment (Table S3). As MoO2(acac)2 resulted in the highest conductivity among the materials producing conductive layers in H2 plasma treatment and, therefore, appeared as the most preferable for S/D electrodes in terms of conductivity, surface analysis was conducted for the MoOx layers. We first confirmed that the H2 plasma-treated MoOx films were amorphous with no apparent micro-grains by XRD and AFM (Fig. S2) irrespective to Tp (up to 300 °C). From UPS analysis, an oxygen defect feature in a lower binding energy region associated with MoO3 with high oxygen vacancy or MoO2 was found (Fig. S3)24. We also estimated work functions at the surface of the samples as 5.1, 5.3, 5.4, 5.4 and 5.6 eV for Tp = 100, 150, 200, 250 and 300 °C, respectively. The result was indeed counterintuitive as a more conductive sample suggesting a higher degree of reduction (lower average oxidation states) is expected in general to lead to a smaller work function. In XPS, a gradual increase in the 3d peaks of Mo (IV) was found up to Tp = 200 °C, which primarily contributes to the increase in conductivity (Fig. S4 and Table S4). However, the contribution from Mo (VI) also increased with Tp in the present case. The reason for the co-presence of Mo (IV) and Mo (VI) at higher Tp is unknown but a possible explanation is that the samples could be re-oxidized during the air-purging step where the heated samples were exposed to oxygen after the H2 plasma treatments. XPS analysis also confirmed that carbon residues remained inside the plasma-treated layers (Fig. S5). As a short summary, (i) the amorphous structure with insufficient reduction toward Mo (IV) can attribute to the obtained resistivity being two orders of magnitude higher than an epitaxial MoO225 and (ii) the complicated feature of oxidation states and carbon residues could lead to the counterintuitive tendency of the work function measured with UPS. We consider that the quality of the printed MoOx can be further improved by optimizing the reduction process flow and adopting a complementary process, such as UV exposure, that efficiently decomposes unwanted carbons26,27.


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Figure 3. Dependence of resistivity of H2 plasma-treated Mo (VI), Co (II) and V (IV) inks on (a) the plasma processing time tp at Tp = 250 °C and (b) the chamber stage temperature Tp for tp = 120 s.

As indium-containing oxides are severely damaged by H2 plasma, prior to the fabrication of an IGZO TFT, candidates for an insulating masking layer for IGZO protection were systematically tested. As a result, Mn (III), Fe and Zr acetylacetonate inks were found to be H2 plasma-resistant and not reducible, while other metals were found to be sputtered away during the H2-plasma treatment, as confirmed by visual inspections and thickness measurements (Table 1)28. Among those elements, the Zr(acac)4 ink was used for the barrier as its oxide has a wide bandgap of 5.8 eV26. The film forming capability, etching resistivity and reducibility against H2 plasma for the various metal (acac) complexes are summarized in Table 1.

Table 1. Solubility, film-forming capability on a PDMS surface, H2-plasma etching resistivity and reducibility to a conductive state of metal (acac) complex inks for reverse offset printing. SS: slightly soluble, S: soluble and VS: very soluble. Y: capable and N: not capable. Metal complexes

VO

Cr

Mn

Fe

Solubility

S

VS

VS

VS

Co (II) VS

Co (III) VS

Ni

Film forming

Y

N

Y

Y

Y

N

Y

H2 plasma resistance at 300 °C for 300 s H2 plasma reduction

Y

Y

Y

Y

N

N

Y

N

N

Y

Cu

Zn

Ga

Zr

MoO

In

SS

SS

S

VS

VS

VS

VS

N

Y

N

Y

Y

Y

Y

Y

N

N

Y

2

N


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In the fabrication of IGZO TFT with the aforementioned patterning technique, an uniform IGZO layer was first formed by a solution process described in SI on a heavily-doped silicon wafer with a 300-nm-thick oxidized SiO2. On the top of the IGZO layer, Zr(acac)4 and MoO2(acac)2 layers were printed by the ROP to form a barrier and S/D electrodes, respectively (Figure 4a). Each layer was preannealed for 5 min at 130 °C and then treated by the H2 plasma at the condition of 1.5 kW, 200 °C and 60 sec. The resulted thicknesses of IGZO, ZrOx, and MoOx layers were 20, 35 and 28 nm, respectively. Figure 4b shows a 6-inch-scale ROP pattern with the bottom-gate top-contact TFTs. Figure 4c shows transfer and output characteristics obtained from the printed TFTs. As the average values of five TFTs, mobility of 0.17 cm2/Vs, subthreshold swing of 0.92 V/dec and Vth of -8.0 V, and on/off ratio ranging from 10-4 to 10-7 were obtained. As a reference, a TFT without a ZrOx layer gave a totally off current curve. A SEM observation confirmed that the IGZO region without the mask layer was deteriorated to a topology of scattered pebbles due to the ion bombardment (Fig. S6). The performance of the present TFTs were inferior to the case of vapor-deposited aluminum S/D electrodes (Fig. S7). The decreased mobility can partly be attributed to the insufficient reduction of the printed molybdenum oxide layer whose work function was 5.4 eV. However, the output curve showing no apparent current crowding indicated another mechanism for the present performance. The high variation among the off currents of the TFTs and the observation of the Vth shift and hysteresis loop in the transfer characteristics suggested that hydrogen doping of the IGZO may be taking place during the H2 plasma treatment. This would lead to the formation of a higher density of both shallow and deep sub-gap traps in the IGZO layer underneath the ZrOx29,30. Moreover, if the same doping effect occurred in the IGZO layer underneath the MoOx layer (in the electrode area), conductive IGZO could possibly behave as a charge transfer layer that contributes to the present device performance. To improve the passivation quality of the upper layers (ZrOx, MoOx) during the H2 plasma, we consider utilization of a complementary processes for enhanced ligand decomposition, such as UV irradiation26,27, and performing optimization of the passivation layer thickness and prebake conditions. In a summary, the present work clearly shows the capability of the high-resolution ROP printing of


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various metal complex precursors that is suitable for electronic device fabrication and enables further integration of fully-printed, multilayered devices.

Figure 4. (a) Schematic illustration of the fabricated TFT structure with spin-coated IGZO, printed MoOx S/D and ZrOx barrier layer. (b) An example of a 6-inch scale ROP pattern of metal (acac) inks. (c) transfer and (d) output curves of a MoOx/ZrOx/IGZO TFT with the channel length of 60 µm and the width of 240 µm.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website Loss rates of the ink solvents by evaporation and PDMS uptake; detailed ink formulations; equilibrium constants of oxygen in various metal oxides; resistivity measurements of ROP-patterned inks; AFM images and XPS/UPS data of H2 plasma-treated MoO2(acac)2; fabrication procedure of the solution-processed IGZO layer; SEM images of the TFT; characteristics of an Al-contact IGZO-TFT (PDF)


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Acknowledgments The authors gratefully acknowledge the technical assistance of Mariko Fujita. Y.K. thanks to JSPS KAKENHI Grant-in-Aid for Young Scientists (B) 17K18410 and Grant-in-Aid for Scientific Research (S) 16H06382. J.L., A.S. and A.A. are supported by the Academy of Finland under project ROXI grant No. 305450. XPS and UPS measurements were performed at AIST Nano-Processing Facility (AISTNPF).

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