Printable poly(methylsilsesquioxane) dielectric ink and its application in solution processed metal oxide thin-film transistors

Abstract

Thermally cross-linkable poly(methylsilsesquioxane) (PMSQ) has been investigated as a printable dielectric ink to make the gate insulator for solution processed metal oxide (IGZO) thin-film transistors by aerosol jet printing. It was found that by increasing the curing temperature from 150 to 200 °C, the dielectric constant and loss tangent of the printed PMSQ layer reduces dramatically. The mobility, leakage current and gate current of the PMSQ enabled thin-film transistor reduces accordingly, while the on/off ratio increases with the increase of curing temperature. An interfacial layer was introduced to further improve the on/off ratio to 3 × 10⁵ and reduce the leakage current to 2.6 × 10⁻¹⁰ A, which is the best result for the solution processed IGZO thin-film transistors using the PMSQ as the gate insulator at a curing temperature of only 150 °C. The study has demonstrated the feasibility of fabricating IGZO thin-film transistors by an all solution-based process.

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1. Introduction

Printable conductors, semiconductors, and dielectric materials are all crucial for the fabrication of large area, flexible and low cost electronics.^1–6 Considerable progress has been made in printable semiconductors for thin film transistors (TFTs), and the carrier mobility that has been achieved within the last few years is at a level comparable to that of hydrogenated amorphous silicon transistors.^7–9 Recently, printable gate dielectrics (polyvinyl phenol^10–15 and polyimide¹⁶) have attracted much attention, as it is well known that the electrical performance of transistors crucially depends on the properties of the gate dielectric materials. However, most of the accessible solution type of gate dielectrics such as poly(methylmethacrylate) (PMMA) have a relatively poor chemical resistance to organic solvent, which greatly restrict their application in the solution processing of TFTs. On the other hand, some thermally curable polymer dielectrics, such as the solution type of polyimide (PI)¹⁷ and polyvinyl phenol (PVP)¹⁸ require much higher curing temperatures (>200 °C) to allow for the use of conventional cheap plastic substrates with glass transition temperatures that are usually lower than 150 °C. Additional cross-linking agents are also required for the cross-linking reactions. In contrast, poly(methylsilsesquioxane) (PMSQ) has recently emerged as the most attractive organic-inorganic hybrid dielectric.^8,9,17,19,20 Firstly, compact PMSQ films can be obtained at a curing temperature as low as 150 °C through the polycondensation reactions of the Si–OH groups without any cross-linking agents; secondly, PMSQ solution can be readily synthesized by a sol–gel method without poisonous and harmful chemicals; thirdly, the surface morphology and dielectric properties of PMSQ films can be easily tuned by altering their organic side groups. Finally, PMSQ is chemically resistant to common organic solvents (such as methylbenzene, alcohol, chloroform), thus making it stable during the solution deposition of organic semiconductors.

Though PMSQ has been widely used in solution processed organic thin-film transistors (OTFTs), these transistors suffered from a high off current, leading to low on/off ratios. In 2008, Yamazaki and his coworkers¹⁷ fabricated poly(3-hexylthiophene)-based (P3HT) field effect transistors, using PMSQ as the gate insulators cured at 150 °C. The fabricated OTFTs exhibited an on/off ratio of less than 10³ and an off current of 10⁻⁸ A. It was suggested that the high off current was caused by the residual silanol groups due to the incomplete polycondensation of PMSQ.²¹ The off current was reduced to 6 × 10⁻¹⁰ A in another study by increasing the curing temperate from 150 to 190 °C in α,ω-dihexylquaterthiophene (DH4T)-based OTFTs.²² Nagase and his coworkers⁹ systematically investigated the synthetic conditions of the PMSQ solution. They found that PMSQ with a low silanol concentration could be obtained by using PGMEA as the solvent during synthesis, and the polycondensation reaction could take place at a temperature of 70 °C. They managed to achieve an on/off ratio of 10⁴ and a leakage current of 3 × 10⁻⁹ A in their P3HT-based TFTs using PMSQ as the gate insulator. Though improved, these values are still not good enough for the practical use of printed TFTs.

While the solution type of organic semiconductors still suffer from low carrier mobility and environmental instability issues, inorganic semiconductors, in particular metal oxide semiconductors, such as indium–gallium–zinc oxide (IGZO), have found their place in TFTs due to their high carrier mobility, high environmental stability, good uniformity and low cost. They have already been used to make backplane driving electronics in flat-panel display applications.²³ High performance IGZO TFTs can be fabricated either through vacuum sputtering deposition or by solution/printing deposition processes.^24–30 For the solution processing of metal oxide TFTs, high performance gate dielectrics are needed, which should ideally be deposited by a printing or solution based process.

In this study, we investigated the electrical properties of PMSQ films that were formed by a solution process and were treated under different curing temperatures and then these dielectric films were applied as gate insulators in IGZO thin-film transistors. An interfacial passivation layer was introduced, prior to the solution deposition of PMSQ, which improved the on/off ratio to 3 × 10⁵ and reduced the leakage current to 2.6 × 10⁻¹⁰ A. This is the best result using solution deposited PMSQ as a gate insulator with a low curing temperature of 150 °C. Top-gate and top-contact IGZO TFTs were also fabricated by printing PMSQ as the gate insulator, instead of spin coating, and the devices showed an on current value up to 3.4 × 10⁻⁵ A, an effective mobility up to 0.75 cm² V⁻¹ s⁻¹, a leakage current 1.66 × 10⁻⁹ A and an on/off ratio of 2 × 10⁴.

2. Experimental section

2.1 Materials

In(NO₃)₃·6H₂O, Ga(NO₃)₃·9H₂O and Zn(NO₃)₂·6H₂O were purchased from Acros. 3-Glycidyloxypropyltrimethoxysilane (GPTMS), formic acid and propylene glycol monomethyl ether acetate (PGMEA) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the solvents were of analytical grade and used without further purification.

2.2 Synthesis of PMSQ ink and the deposition of PMSQ films

Poly(methylsilsequioxane) (PMSQ) solution was synthesized according to the previously reported procedure.⁹ Deionized water with an acid catalyst was slowly added to the methyltrimethoxysilane PGMEA solution. The mixture was stirred for 30 min at room temperature and then heated up to 70 °C for 1 h to initiate the polycondensation reaction. A highly viscous PMSQ solution was obtained after vacuum distillation at 70 °C to remove any small molecules and then it was diluted with PGMEA to the designated concentration (0.45 g mL⁻¹). For the preparation of PMSQ ink, the viscous PMSQ was dissolved in acetone to obtain a concentration of 15 mg mL⁻¹. PMSQ films were formed by either spin coating (2000 rpm, 60 s) or aerosol jet printing (M3D-103, Optomec), followed by curing in a muffle furnace at the designated temperatures for one hour in air.

2.3 Fabrication of metal oxide TFTs

Two types of IGZO-based TFTs were fabricated according to the reported procedure.³¹ One was a bottom-gated TFT using SiO₂ as the gate insulator and another was the top-gated TFT using solution deposited PMSQ as the gate insulator. For the bottom-gated TFT, the IGZO film was prepared by spin coating the precursor solution on a silicon wafer with 300 nm thick thermally grown SiO₂, followed by annealing at 450 °C in air for 60 min. The IGZO precursor solution was prepared by dissolving In(NO₃)₃·6H₂O, Ga(NO₃)₃·9H₂O and Zn(NO₃)₂·6H₂O (molar ratio In : Ga : Zn = 3 : 1.5 : 2) with ethanolamine in 5 mL 2-methoxyethanol. The final concentration of metallic salts was 0.05 mol L⁻¹. The source/drain electrodes were made by thermally evaporating 120 nm thick Al onto the IGZO film via a shadow mask. The heavily doped silicon substrate served as the gate electrode. The channel length (L) and width (W) were 50 and 1000 μm, respectively. Moreover, a surface modification of the IGZO film was carried out by immersing the transistor into 0.5 mM GPTMS solution in toluene (with 4.5 mM water) for 24 h. For the top-gated TFT, a PMSQ layer was deposited on top of the IGZO film either by spin-coating or aerosol jet printing. Al gate electrode was thermally evaporated on top of the PMSQ film, in addition to the Al source/drain electrodes as described above. For the dielectric characterisation of PMSQ films, an Al–PMSQ–ITO structure was utilized. The PMSQ films were prepared by spin-coating the precursor solution onto ITO glass. Al electrodes were then deposited on top of the spin-coated PMSQ films by thermal evaporation.

2.4 Characterization

The wettability of the surfaces was characterized via a static water contact angle goniometry at room temperature (Kino, SL200C). The transmittances of the glass substrate, glass/IGZO, glass/PMSQ and glass/IGZO/GPTMS/PMSQ were characterized using a UV-vis spectroscope (Perkin Elmer Lambda 750 spectrometer). The frequency-dependent loss tangent (dissipation factor) of PMSQ films was measured by a Keithley 4200 Capacitance Voltage Unit (from 10 kHz to 1 MHz at 6 V) at room temperature. A step profiler (Veeco, Dektak 150) was used to measure the thicknesses of the PMSQ films. The thickness of PMSQ films for the measurement of dielectric constant and leakage current density was 2 ± 0.2 μm. The dielectric constants were calculated from the measured capacitances, thicknesses and areas of the top electrode.¹ At least seven individual measurements were made to determine the dielectric parameters of PMSQ films. All the TFTs were characterized by a Keithley Instruments Model 4200-SCS in atmospheric conditions. The saturation mobility was calculated from formula (1) as follows:where C_i is the capacitance of the gate dielectrics per unit area, W and L are the channel width and length, respectively, V_GS is the gate voltage, and I_D is the drain current.

3. Results and discussion

The high temperature curing of PMSQ is necessary to ensure the complete reaction of silanol groups in the PMSQ films. There are few reports on the electrical properties of PMSQ film cured at temperatures above 250 °C. As can be seen in Fig. 1, the spin-coated PMSQ film chapped if directly cured at 450 °C on a hotplate due to internal stress, as seen in Fig. 1a. A heating ramp at a rate of 5 °C min⁻¹ in a muffle furnace eliminated the cracks, as seen in Fig. 1b.

Fig. 1 The morphology of the PMSQ film cured (a) directly on 450 °C hotplate, (b) a ramp up heating at the rate of 5 °C min⁻¹ in a muffle furnace for 1 h.

Table 1 and Fig. 2 show the leakage current density, dielectric constant and loss tangent of the PMSQ films cured at different temperatures, which were measured with the Al–PMSQ–ITO (metal–insulator–metal) structure. As the curing temperature increased, all the three parameters of the films decreased accordingly. Note that with 20 V μm⁻¹ and loss tangent (<0.005) over a frequency range of 10 kHz to 1 MHz, the films cured at 350 and 450 °C, thus showing very low leakage current densities (<10 nA cm⁻²). This is attributed to the extremely low concentration of polar silanol groups and residuals of small molecules in the high temperature treated PMSQ films. Simultaneously, the dielectric constant decreased with the low content of polar groups and molecules, as seen in Table 1, due to the decrease of polarizability.

Table 1 The permittivity and leakage current density of the PMSQ film at different curing temperatures

Curing temperature (°C)	Leakage current density^a (nA cm^−2)	Dielectric constant (10 MHz)
a Applied voltage is 20 V μm^−1.
150	48–80	4.0
250	24–40	3.0
350	8–32	2.5
450	<10	2.4

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Fig. 2 Frequency dependence of loss tangent for the PMSQ films cured at different temperatures.

PMSQ has been used as a dielectric layer in organic thin-film transistors in the past, but suffered from low on/off ratios and a high off current.^{8,21,22,32–34} Jeong and his coworkers found that increasing the curing temperature of a PMSQ-insulator could increase the on/off ratio and reduce the leakage current.³⁴ A similar trend has been found for inorganic TFTs. Fig. 3 shows the transfer characteristics of top-gate top-contact IGZO-TFTs with spin-coated PMSQ dielectric layers cured at 150 and 200 °C. The inset in Fig. 3a is the schematic of the device. The 150 °C cured PMSQ-insulator TFT (Fig. 3a, Table 2) showed an on current up to 2 × 10⁻⁴ A, saturation off current in the range 10⁻⁶ to 10⁻⁷ A and an on/off ratio of 2.5 × 10³ when the voltage between the drain and the source (V_DS) was 40 V. When the PMSQ-insulator curing temperature increased to 200 °C (Fig. 3b, Table 2), the on/off ratio increased to 5 × 10⁴ and the off current decreased to 0.8–5 × 10⁻⁹ A. Unfortunately, the mobility was also reduced from 1.8 cm² V⁻¹ s⁻¹ at 150 °C to 0.4 cm² V⁻¹ s⁻¹ at 200 °C curing. The mobility degradation was due to the poor contact between the Al electrode and the IGZO semiconductor from the high curing temperature. Further increasing the curing temperature of PMSQ (250, 350 and 450 °C) resulted in damaging the Al electrodes. We also fabricated PMSQ-insulator TFTs on a glass substrate (see ESI, Fig. S1†), and it showed an on current of up to 3.8 × 10⁻⁴ A, a mobility of 0.7 cm² V⁻¹ s⁻¹ and an on/off ratio of 8.5 × 10⁴, while the thickness of the PMSQ (200 °C) was 600 nm.

Fig. 3 (a) Transfer characteristics of IGZO TFTs with spin-coated PMSQ dielectric layer cured at a temperature of (a) 150 °C; (b) 200 °C (the inset image is the schematic cross-sectional view of a fabricated top-gate, top-contact PMSQ-insulator IGZO TFT).

Table 2 The characteristics of PMSQ-insulator IGZO TFTs

Curing temperature (°C)	Mobility (cm^2 V^−1 s^−1)	On/off ratio	Leakage current (10^−9 A)
a IGZO and Al electrodes modified with GPTMS.
150	1.8	2.5 × 10^2	100–1000
200	0.4	5 × 10^4	0.8–5
150^a	1.0	3 × 10^5	0.26

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Fig. 4 shows the transfer characteristics of top-gate, top-contact IGZO transistors before and after coating the PMSQ layer. The results indicate that the PMSQ layer itself does not increase the leakage current between the source and drain electrodes as seen in Fig. 4a, which is in the range of 10⁻¹⁰ to 10⁻¹¹ A with the gate voltage varied from −40 to 0 V. However, when there is an Al electrode on top of the PMSQ layer, the leakage current increased sharply to 10⁻⁸ A when the gate voltage was −30 V, as seen in Fig. 4b. In fact, the high leakage current I₁ (Fig. 4b) has included the contribution of current from the drain electrode to the top Al electrode and to the Al source electrode.

Fig. 4 Transfer characteristics of the IGZO-based SiO₂-insulator transistor with (a) PMSQ, (b) PMSQ/Al electrode.

In order to reduce the leakage current, a molecular passivation using GPTMS on both the Al electrode and the IGZO films was introduced prior to the deposition of the PMSQ layer. By immersion into the GPTMS solution, the organic molecules were self-assembled onto Al and IGZO surfaces through the hydrolysis of the triethoxy groups of GPTMS and condensation of the hydroxyl groups in GPTMS, as shown in Fig. 5a. AFM images show that the root mean square roughness (R_q) of the IGZO films increased from 0.46 nm to 0.74 nm after GPTMS modification (see ESI, Fig. S2†), indicating the formation of a dense GPTMS layer on the IGZO films.⁶ It has been experimentally proven that the GPTMS modification would not change the performance of IGZO-based TFT in the mobility, threshold voltage, off/on current and subthreshold swing, as shown in Fig. 5b, because there is no polar group in the GPTMS molecule.

Fig. 5 (a) Schematic illustrations of GPTMS modified IGZO TFT structure showing the chemical binding of GPTMS to IGZO. (b) Transfer characteristics of IGZO TFTs before and after the GPTMS treatment.

In contrast, other silane coupling agents were also applied to modify the IGZO surface. A dramatic reduction in the threshold voltage was noticed when an agent with a polar group was used such as (3-aminopropyl)triethoxysilane (APTES, see ESI, Fig. S3†). A similar modification was also carried out by Seong and his coworkers. It was found that the APTES molecules self-assembly led to the n-type doping of ZnO because of the strong electron-donating characteristics of the amine group, and finally the threshold voltage and mobility of ZnO TFTs changed accordingly.⁶ Agents that contain Si–Cl groups, such as phenyl trichlorosilane (M1) and 1H,1H,2H,2H-perfluorodecyltrlchlorosilane (M2), will cause great damage to the Al electrodes and IGZO film (see ESI, Fig. S4†). This is because of a large quantity of HCl was generated during the hydrolysis of M1 and M2.

After GPTMS modification, PMSQ ink was spin-coated and cured at 150 C, followed by the deposition of the Al gate electrode. The final TFT structure is shown in Fig. 6a. The GPTMS modified TFT showed an on current value of up to 7.9 × 10⁻⁵ A, a leakage current 2.6 × 10⁻¹⁰ A, an on/off current ratio of 3 × 10⁵, and an effective mobility up to 1.0 cm² V⁻¹ s⁻¹, as seen in Table 2. Fig. 6b is the transfer curve of the TFT. The output characteristics are shown in Fig. 6c, which demonstrates a typical n-channel transistor with a clear pinch-off and excellent saturation, implying that the entire thickness of the IGZO channel can be completely depleted of free electrons. Compared to the TFT without modification (Fig. 3), the modified TFT had a much lower gate and leakage currents, a higher on/off ratio and acceptable mobility. There are two possible explanations for the improvement. Firstly, the GPTMS molecules form a passivation layer both on the Al and IGZO surfaces. The conductive path from the source/drain electrodes and IGZO to gate electrode is blocked. Secondly, the contact angle of IGZO and Al electrodes reduced from 88° and 83° to 66° after GPTMS treatment. The change of surface energy may lead to better quality of PMSQ deposition, which helps to reduce the leakage current.

Fig. 6 (a) The schematic cross-sectional view of a fabricated GPTMS modified bottom gate, top contact PMSQ-insulator IGZO TFT. (b) Transfer characteristics and (c) output characteristics of IGZO TFTs (curing temperature is 150 °C).

Fig. 7a shows the image of IGZO-based TFT, which has GPTMS modification and printed PMSQ ink as the dielectric layer. The TFT showed an on current value up to 3.4 × 10⁻⁵ A, a leakage current of 1.66 × 10⁻⁹ A, an on/off ratio of 2 × 10⁴, and an effective mobility value up to 0.75 cm² V⁻¹ s⁻¹, as can be seen in Fig. 7b. Compared to the TFT with spin-coated PMSQ, its performance is slightly reduced. It may be due to the thicker printed PMSQ film (1.8 μm) than the spin-coated one (1 ± 0.2 μm). It is difficult to control the thickness of the printed PMSQ layer by using the PMSQ-insulator ink with the present formulation method. The PMSQ ink needs to be further improved for printing thin layers.

Fig. 7 (a) The image and (b) transfer characteristics of GPTMS modified, printed PMSQ-insulator IGZO-based TFT (PMSQ curing temperature is 150 °C).

To the best of our knowledge, the results of TFTs fabricated by PMSQ-insulator with such a low leakage current and a high on/off ratio have not been reported until now, if the curing temperature was no higher than 190 °C.²² P4VP insulator, the most popular solution processable dielectric used in the fabrication of transistors, has a leakage current of 2 × 10⁻¹⁰ to 10⁻⁸ A.^10,35–39 As mentioned above, the comparatively large leakage current of the PMSQ-insulator transistor has greatly restricted the dielectrics application of PMSQ. After GPTMS modification, the leakage current of PMSQ insulator has a similar magnitude equal to PVP.

In addition, IGZO, PMSQ and IGZO/GPTMS/PMSQ films on the glass substrate showed a very high transparency (about 95%) in the visible region (400–800 nm wavelength) with no apparent drop compared to the bare glass substrate. The high transparency of IGZO/GPTMS/PMSQ indicates their potential for transparent and high-performance electronics (see ESI, Fig. S5†).

4. Conclusions

The thermally cross-linked poly(methylsilsesquioxane) (PMSQ) was investigated as a printable dielectric ink and as a gate insulator in IGZO-based thin-film transistors by solution deposition under different curing temperatures. It was found that the dielectric constant, leakage current density and loss tangent of the PMSQ layer reduced from 4.0, 48–80 nA cm⁻² and 0.015 to 2.4, <10 nA cm⁻² and 0.0025, respectively, when the curing temperature was increased from 150 to 450 °C. When employing the PMSQ layer as a gate insulator in IGZO based TFTs, an interfacial passivation layer of GPTMS was introduced, prior to solution deposition of PMSQ, which improved the on/off ratio to 3 × 10⁵ and reduced the leakage current to 2.6 × 10⁻¹⁰ A. This is the best result using solution deposited PMSQ as gate insulator with low curing temperature of 150 °C. Top-gate and top-contact IGZO TFTs were also fabricated by printing PMSQ as the gate insulator, instead of spin coating, and the devices showed an on current of up to 3.4 × 10⁻⁵ A, an effective mobility up to 0.75 cm² V⁻¹ s⁻¹, a leakage current 1.6 × 10⁻⁹ A and an on/off ratio of 2 × 10⁴.

Acknowledgements

This study was supported by the project of the Major Research plan of the National Natural Science Foundation of China (Grant no. 91123034) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA09020201).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra17234e

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