OXIDE SEMICONDUCTOR thin-film transistors (TFTs)

Transcription

1 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 1, JANUARY Effect of Self-Assembled Monolayer (SAM) on the Oxide Semiconductor Thin Film Transistor Seung-Hwan Cho, Yong-Uk Lee, Jeong-Soo Lee, Kang-Moon Jo, Bo Sung Kim, Hyang-Shik Kong, Jang-Yeon Kwon, and Min-Koo Han Abstract In this paper, we proposed the self-assembled monolayer (SAM) as a protection layer against plasma and chemically induced damages to the back interface of an oxide semiconductor during the deposition of the passivation layer. When a thin-film transistor (TFT) is passivated with plasma-enhanced chemical-vapor deposition (PECVD) SiO and solution-based materials, the back interface of the oxide semiconductor could be exposed to plasma and chemically induced damages, respectively. We employed SAMs on the back surface of the oxide semiconductor prior to the passivation process to suppress such damage. The hydrophobic Cl-SAM (3-chloropropyltriethoxysilane) suppressed the degradation in mobility and subthreshold slope (SS) due to ion bombardment during plasma treatment. The hydrophobic CH 3 -SAM (octyltriethoxysilane) successfully blocked chemically induced damage due to solution-based passivation. Index Terms Back interface, oxide semiconductor, self-assembled monolayer (SAM), solution-based passivation. I. INTRODUCTION OXIDE SEMICONDUCTOR thin-film transistors (TFTs) have attracted considerable attention for large size displays, because the oxide semiconductors, such as IGZO [1] [3] and ZTO [4] [6], exhibit high carrier mobility, even in an amorphous state and have good uniformity. However, oxide semiconductor TFTs are very sensitive to environments such as moisture and oxygen. It has been reported that the gaseous molecules are strongly associated with characteristics of oxide transistors [7] [9]. The oxide semiconductor should be passivated with a dense passivation material, such as SiO to block these molecules. SiO deposited by plasma-enhanced chemical-vapor deposition (PECVD) is widely used in passivation materials of oxide semiconductors. In addition, solution-based passivation materials are under development for the printing process. However, TFT characteristics can be altered due to plasma damage and chemically induced damage caused by the passivation mentioned above. The oxide semiconductor is exposed to various plasma, including hydrogen, nitrogen, and oxygen radicals, during SiO passivation by PECVD. It is well known that plasma can degrade TFT characteristics due to ion bombardment. The IGZO film becomes highly conductive by the reduction of oxygen due to the incorporation of hydrogen into the oxide semiconductor [10]. In addition, when the oxide semiconductor is exposed to organic solvents, the adsorbed solvent molecules with high polarity on the back interface of the oxide semiconductor induce carriers in the back surface of the channel and thus threshold voltage is shifted to the negative direction [11]. Therefore, the passivation on the back interface of the oxide semiconductor should be considered to protect the back channel from plasma damage and chemically induced damage because the reliability of the TFT is critical to the back channel. The purpose of the paper is to report that SAM successfully suppresses physical and chemical damages on the back interface of the oxide semiconductor as a protection layer. We have deposited SAM on the back surface of the oxide semiconductor prior to passivation. SAMs are easily formed by the chemisorption of hydrophilic head groups on a substrate packed closely to each other by van der Waals interactions [12]. SAM can be applied widely in a passivation layer, since a close-packed SAM is resistant to chemical and physical damages. In this paper, we investigated SAM as a protection layer against physical and chemical damages caused by passivation on the back interface of an oxide semiconductor. Manuscript received April 29, 2011; revised August 17, 2011; accepted September 19, Date of publication November 14, 2011; date of current version January 04, S.-H. Cho is with the Department of Electrical Engineering and Computer Science, Seoul National University, Seoul , Korea, and also with the R&D Center, LCD department, Samsung Electronics, Gyeonggi-do , Korea. ( sh2020.cho@samsung.com). Y.-U. Lee, J.-S. Lee, and M.-K. Han are with the Department of Electrical Engineering and Computer Science, Seoul National University, Seoul , Korea ( leeyu@emlab.snu.ac.kr; jslee@emlab.snu.ac.kr; mkh@snu.ac. kr). K.-M. Jo, B. S. Kim, and H.-S. Kong are with the R&D Center, LCD department, Samsung Electronics, Gyeonggi-do , Korea. ( kangmoon.jo@samsung.com; bskim86@samsung.com; hskong@samsung.com). J.-Y. Kwon is with the School of Integrated Technology, Yonsei University, Incheon , Korea ( jangyeon@yonsei.ac.kr). Color versions of one or more of the figures are available online at ieeexplore.ieee.org. Digital Object Identifier /JDT II. EXPERIMMENTAL We fabricated bottom gate a-gizo TFTs, as shown in Fig nm Mo metal is deposited on a glass substrate as a gate electrode. SiO (300 nm) was deposited by PECVD at 370 Cas a gate insulator. A 40 nm ITO metal is used as drain and source electrodes. The a-igzo active layer was formed by sputtering. Each layer is patterned by photolithography and etching. Then, we deposited two kinds of functionalized SAMs (3-chloropropyltriethoxysilane (Cl-), octyltriethoxysilane (CH -)) on the oxide TFT by the solution method prior to passivation [13]. We employed various widely used materials, such as PMMA, SiO, and SiN, as passivation layers. PMMA is spin-coated by the solution method and annealed at 160 C for 20 min SiO with N O treatment is deposited at 150 C and SiN without X/$ IEEE

2 36 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 1, JANUARY 2012 Fig. 1. Cross section of fabricated a- IGZO TFT. Fig. 2. V (10 pa) shift of TFT with various SAMs before and after SAM treatment ( 1V (10 pa) = V (10 pa) (after SAM treatment) V (10 pa) (before SAM treatment). pretreatment is deposited at 180 to 2000 Å film thickness. C by PECVD, respectively, III. RESULTS AND DISCUSSION A. Transfer Characteristics of a-igzo TFT With SAM Treatment After SAM treatment, we observed the different turn-on voltage ( )( at pa) shift of oxide TFTs with two different SAM treatments, as shown in Fig. 2 ( (10 pa) (after SAM treatment) (10 pa) (before SAM treatment)). The in Cl-SAM treated TFT was shifted to the negative direction, while that for in CH -SAM treated TFT varied in the range of V to 0.4 V. This phenomenon is due to SAM having a dipole moment according to a functional group [14] [17]. The dipole moment is negative in the Cl-functional group [16], while it is positive in the CH -functional group [17] (in this paper, the positive direction in the dipole moment is same with that of electric field toward the back interface of the oxide semiconductor). Thus, each SAM can generate the built-in local electric field that accumulates or depletes electrons on the oxide semiconductor. Cl-SAM with negative dipole moment creates a local electric field normal to the semiconductor [16]. This electric field accumulates electrons near the back interface of the oxide semiconductor. Therefore, Cl-SAM with a negative dipole moment forms a high conductive back channel which requires more negative gate bias to deplete the active channel layer. In contrast, CH -SAM with a positive dipole moment generates the opposite build-in electric field to Cl-SAM with a negative dipole moment [16]. The magnitude of the shift in CH -SAM treated-tft was relatively small compared to Cl-SAM treated-tft. In addition, the direction of the shift did not indicate any significant meaning, as Fig. 3. Transfer characteristics of a-igzo TFT as a function of applied gate bias stress time for the (a) TFT(A) with Cl-SAM. (b) TFT(B) without SAM. shown in Fig. 2. It may be attributed to the fact that the number of holes are much less than that of electrons in n-type oxide semiconductor such as IGZO and the built-in electric field due to CH -SAM is not large enough to induce carriers [17]. In that case, the electric field due to CH -SAM with a positive dipole moment could not sufficiently induce holes near the back interface of the oxide semiconductor. Therefore, the small number of induced holes near the back interface can scarcely bend the band. On the other hand, the electric field due to Cl-SAM with a negative dipole moment sufficiently accumulates electrons near the back channel and thus can easily bend the band. This agrees fairly well with the shift in TFT with CH -SAM treatment being negligible. In addition, we measured the bias stability of a-igzo TFTs to investigate SAM as a blocking layer from moisture. The experiments were performed at air condition (40% relative humidity). We observed similar positive shift in both TFT (A) with Cl-SAM and TFT (B) without SAM under positive gate bias stress ( V, V, from 0 s to 1000 s) (the result is not shown here). In contrast, the virgin TFT (B) (without SAM treatment) exhibits large negative shift, including a change of subthreshold slope (SS) compared to TFT (A) with Cl-SAM treatment under negative gate bias stress ( V, V, from 0 s to 1000 s), as shown in Fig. 3. It was previously reported that threshold voltage is

3 CHO et al.: EFFECT OF SAM ON OXIDE SEMICONDUCTOR TFT 37 Fig. 4. Schematic image of field-induced adsorbed H O molecules on the back interface of oxide semiconductor. (a) virgin TFT. (b) TFT with SAM treatment. Fig. 5. Change of mobility and subthreshold swing (SS) before and after SiOx passivation (1SS = SS (after SAM treatment) SS (before SAM treatment)). TABLE I ELECTRICAL CHARACTERISTICS OF OXIDE TFTS. shifted in the negative direction and SS is degraded under negative bias stress, because the field-induced O molecules on the back surface of the oxide semiconductor can act as electron donors as well as acceptorlike deep traps [7]. However, the surface of the oxide semiconductor in Cl-SAM treated TFT is much more hydrophobic than in virgin TFT due to the adsorbed SAM on the back surface of the oxide semiconductor. Fig. 4 shows the schematic image of field-induced adsorbed O molecules on the back interface of oxide semiconductor in TFTs with and without SAM treatment, respectively. Larger negative shift in virgin TFT was observed, since the number of field-induced adsorbed O molecules in SAM treated TFT is much fewer than in the virgin TFT. Therefore, we expect the hydrophobic Cl-SAM successfully blocks the adsorption of these field-induced O molecules. B. Transfer Characteristics of a-igzo With SiO or SiN Passivation First, we investigated SAM, as a protection layer that protects the active layer from plasma damage and hydrogen incorporation during the inorganic passivation process. After SAM treatment, we employed the widely used SiO and SiN film for mass production by the PECVD method on the oxide semiconductor. SiO with N O treatment was deposited at 150 C and SiN without pretreatment was deposited at 180 C, respectively, to 2000 Å film thickness. We observed the variation of mobility and subthreshold swing before and after SiO deposition. Table I shows the change of mobility and SS before and after SiO passivation. The mobility in SAM treated TFT decreased from 6.53 cm V s to 5.24 cm V s, while that in virgin TFT decreased from 6.12 to 4.30cm V s as shown in Fig. 5. The change of SS in Fig. 6. Change of mobility and subthreshold swing (SS) before and after the CF plasma treatment (1SS = SS (after SAM treatment) SS (before SAM treatment)). SAM treated TFT was 0.58 V/decade, while that in virgin TFT was 1.1 V/decade. When the back interface of the oxide semiconductor is exposed to plasma, a damaged bond can be created in the active layer by ion bombardment and it can act as a defect site [10]. Thus, mobility decreases due to damaged bond and SS increases due to defect creation by ion bombardment respectively during plasma treatment. In addition, the adsorbed SAM, as a very thin organic layer on the oxide semiconductor, is resistant to physical damage such as ion bombardment. We treated CF plasma on TFTs after SAM deposition to further investigate the effect of plasma treatment on a-igzo. This is quite similar to the behavior of SiO -passivated TFT before and after passivation, as shown in Fig. 6. Based on this fact, we can explain SAM may effectively suppress physical plasma damage, such as ion bombardment and then the degradation in mobility and SS of SAM-treated TFT is less than in virgin TFT. In contrast, after SiN passivation, we observed all TFTs (from 1 Ato 1.5 Aat V, V) were not modulated by gate voltage (always on-state), even though they were treated with SAM. Similarly, all TFTs became highly conductive after H plasma treatment on a-igzo (the result is not shown here). This is well known as an incorporation of hydrogen provided from SiH and NH into a-igzo film [10]. This result indicates that the main cause of high conductivity of a-igzo during SiN passivation is the oxygen reduction by hydrogen. SAM cannot effectively block the incorporation of hydrogen into a-igzo, in contrast to the suppression of plasma damage, such as ion bombardment.

4 38 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 1, JANUARY 2012 Fig. 7. Relationship between CA and V PMMA deposition. variation of TFT before and after Fig. 8. Relationship between CA and V variation of TFT with negative bias stress (at V = 020 V, V =1Vfor 1 h). C. Transfer Characteristics of a-igzo After Solution-Based Passivation The back interface of an oxide semiconductor is easily exposed to various organic solvents, such as chlorobenzene, hexane, and acetonitrile during solution-based passivation. In this paper, we employed poly-methyl methacrylate (PMMA) that is widely used in the solution-based passivation process. PMMA was spin coated on TFTs and thermally annealed at 160 C for 20 min in air ambient. We measured the current voltage characteristics of the TFTs under dark and air. After PMMA passivation, negative shift was observed in all the TFTs and the magnitude of shift in SAM-treated TFT was much less than in virgin TFT. in CH -SAM treated TFT was shifted from 0.8 V to 6.6 V before and after passivation, while in virgin TFT was shifted from 2.0 V to 12.6 V before and after passivation. This experimental result indirectly indicates that the hydrophobic SAM on the surface of an oxide semiconductor suppresses chemically induced change by PMMA solution. The PMMA solvent is chlorobenzene. It has been reported that gaseous molecules, including chlorobenzene, are absorbed or desorbed on the oxide semiconductor and they change electric properties of the oxide semiconductor TFT [7], [11]. They can easily accumulate electrons near the back interface of the oxide semiconductor. Especially, in the case of solvent molecules, there is a close relationship between shift of TFT and dielectric constant of molecules [11]. When the adsorbed solvent molecules with higher dielectric constant have higher polarity and higher electronegativity, they induce carriers near the back interface of the oxide semiconductor compared to solvent molecules with lower dielectric constant (lower polarity). It was suggested in the paper that the adsorption of high-polar solvent molecules caused more band bending near the back channel surface. Therefore, it was considered that Chlorobenzene with high polarity, the solvent of PMMA solution, is the main cause for the shift in our experiments. Chlorobenzene in the PMMA passivation layer can remain after hardbake (160 C, 20 min). We measured the contact angle (CA) of each TFT, since CA represents the area density of deposited SAM on the surface, as a criterion of hydrophobicity or hydrophilicity to confirm the relationship between shift and SAM. If CA is larger, more of the surface is hydrophobic. The surface of SAM-treated TFT with a large CA is more hydrophobic than is virgin TFT. We believe that SAM on TFT with higher CA is denser and more solid than in the virgin TFT and blocks the adsorption of organic solvent, Chlorobenzene, considerably. Fig. 7 demonstrates the relationship between the CA of each TFT with various SAMs and the corresponding variations. TFT with higher CA caused less negative shift. In addition, we investigated the stability of oxide semiconductor TFT under gate bias stress after PMMA formation, because the remaining solvent in PMMA is adsorbed due to the electric field. It can affect the behavior of. Fig. 8 shows shifts for various TFTs with negative bias stress. Under negative bias stress (at V, V for 1 h), shift in SAM-treated TFT was less than in virgin TFT and there was a variation of shift for each SAM-treated TFTs with different contact angle. This is quite similar to the shift mechanism of the adsorption of chlorobenzene molecule discussed above. We observed that is shifted in a negative direction due to PMMA solvent and the remaining solvent in PMMA can affect characteristics of TFT, even though TFT was hardbaked (160 C, 20 min) after PMMA formation. The chemically induced change can be suppressed by employing SAM on the back surface of an oxide semiconductor. IV. CONCLUSION We investigated the effects of self-assembled monolayer (SAM), as a protection layer of an oxide semiconductor against plasma and chemically induced damages during the deposition of the passivation layer. When TFT is passivated with PECVD SiO and solution-based materials, plasma and chemically induced damages on the back interface of the oxide semiconductor cannot be avoided. However, the hydrophobic Cl-SAM (3-chloropropyltriethoxysilane) suppressed the degradation of TFT characteristics, such as mobility and SS, due to damaged bonds and defect creation by ion bombardment during plasma treatment. The hydrophobic CH -SAM (octyltriethoxysilane) blocked the adsorption of PMMA solvent and thus suppressed chemically induced damage caused by solution-based passivation. In addition, there is no additional photolithographic process for SAM deposition and patterning. Therefore, close-packed hydrophobic SAM can be a promising buffer layer prior to passivation to achieve greater stability.

5 CHO et al.: EFFECT OF SAM ON OXIDE SEMICONDUCTOR TFT 39 REFERENCES [1] K. Nomura, H. Ohta, A. Takaki, T. Kamiya, M. Hirano, and H. Hosono, Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors, Nature, vol. 432, p. 488, [2] J. Park, I. Song, S. Kim, S. Kim, C. Kim, J. Lee, H. Lee, E. Lee, H. Yin, K. K. Kim, K. W. Kwon, and Y. Park, Self-aligned top-gate amorphous gallium indium zinc oxide thin film transistors, Appl. Phys. Lett., vol. 93, p , [3] M. Kim, J. H. Jeong, H. J. Lee, T. K. Ahn, H. S. Shin, J. S. Park, J. K. Jeong, Y. G. Mo, and H. D. Kim, High mobility bottom gate InGaZnO thin film transistors with SiO etch stopper, Appl. Phys. Lett., vol. 90, p , [4] S. J. Seo, C. G. Choi, Y. H. Hwang, and B. S. Bae, High performance solution-processed amorphous zinc tin oxide thin film transistor, J. Phys. D: Appl. Phys., vol. 42, p , [5] Y. J. Chang, D. H. Lee, G. S. Herman, and C. H. 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Mandoc, T. Van Woudenbergh, and P. W. M. Blom, Tuning of metal work functions with self-assembled monolayers, Adv. Mater., vol. 17, no. 5, p. 621, [16] Y. Jang, J. H. Cho, D. H. Kim, Y. D. Park, M. Hwang, and K. W. Cho, Effects of the permanent dipoles of self-assembled monolayer-treated insulator surfaces on the field-effect mobility of a pentacene thin-film transistor, Appl. Phys. Lett., vol. 90, p , [17] S. Kobayashi, T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, and T. Mitani, Control of carrier density by self-assembled monolayers in organic field-effect transistors, Nature Mater., vol. 3, p. 317, Yong-Uk Lee received the B.S. and M.S. degrees in electrical engineering and computer science from Seoul National University, Seoul, Korea, in 2009 and 2011, respectively. His current research interests are solution processed oxide thin-film transistors and driving circuits for active matrix liquid crystal display (AMLCD) and active matrix organic light-emitting diode (AMOLED) Jeong-Soo Lee received the B.S. degree in electrical engineering from Yonsei University, Seoul, Korea, in 2008, and is currently working toward the Ph.D. degree with unified M.S. and Ph.D. degrees in electrical engineering and computer science from Seoul National University, Seoul, Korea. His current research interests are solution processed oxide thin-film transistors and driving circuits for active matrix liquid crystal display (AMLCD) and active matrix organic light-emitting diode (AMOLED) Kang-Moon Jo received the B.S. degrees in physics from the Department of Physics, Konkuk University, Seoul, Korea, in He joined Samsung LCD R&D center in His current research interests are physics of oxide semiconductor TFT and new display technology. Bo Sung Kim received B.S. from Pusan National University, M.S. from KAIST, and Ph.D. in chemistry from Seoul National University, in 1990, 1992, and 2000, respectively. He joined Samsung Electronics Co, Ltd. (SEC) in He has worked the advanced TFT process developments for high-end LCD applications with large size, high resolution, and high aperture. Since 2006, he is a principal engineer at LCD R&D center, SEC, being in charge of the project of solution-processed TFT with oxide semiconductors. His current research interests are organic TFTs, printed electronics, and flexible electronics in display applications. Seung-Hwan Cho received the B.S. degree in material science and engineering from Korea University, Seoul, Korea, in 200, and is currently working toward the M.S. degree in electrical engineering and computer science from Seoul National University, Seoul, Korea. He joined LCD department, Samsung Electronics, Kyunggi-do, Korea, as an engineer in From 2005 to 2009, he had performed the researches regarding the design and characterization of organic TFTs and solution-based inorganic TFTs for LCD display. His research interests are solution-based semiconductor and solution-based passivation. Hyang-Shik Kong received the B.S. degree in physics from Seoul National University, Seoul, Korea, in 1985 and the M.S. and Ph.D. degrees in physics from Korea Advanced Institute Science and Technology, Daejeon, Korea, in 1988 and 1996, respectively. He is a Vice President and a Leader of process development Group with the LCD Research and Development Center, SAMSUNG Electronics, Gyeonggi-do, Korea, where he has been working since His current research interests are advanced TFT process developments, printed electronics, and printing technologies for active matrix liquid crystal display (AMLCD).

6 40 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 1, JANUARY 2012 Jang-Yeon Kwon received the B.S. and M.S. degrees in Metallugical Engineering from Seoul National University, Seoul, Korea, in 1997 and 1999, respectively. He received Ph.D. degree in material science and engineering from Seoul National University in He joined Samsung Advanced Institute of Technology (SAIT), Gyunggi-do, Korea, as a principal researcher, where he had performed the research regarding characteristics of Si and oxide semiconductor thin film transistor. He is currently a professor with the School of Integrated Technology, Yonsei University, Incheon, Korea, from His current interests include the novel oxide semiconductor materials and devices for next generation active matrix display applications. Min-Koo Han received the B.S. degree in electrical engineering from Seoul National University, Seoul, Korea, in 1971 and the Ph.D. degree in electrical engineering from The Johns Hopkins University, Baltimore, MD, in From 1979 to 1984, he was an Assistant Professor with the Department of Electrical and Computer Engineering, State University of New York, Buffalo. Since 1984, he has been with Seoul National University, Seoul, where he is currently a Professor with the Department of Electrical Engineering. His current research interests are amorphous and polycrystalline silicon, oxide semiconductors, and power semiconductor devices.