Search

Because of their superior performances to amorphous silicon (a-Si) semiconductors, amorphous oxide semiconductors have been attracting considerable attentions as backplanes for liquid crystal displays (LCDs) and organic light emitting diode displays (OLEDs). For mass production, the back channel etch (BCE)-type TFTs have considerable advantages owing to lesser process steps, lesser parasitic capacitances and shorter channel length than those of the etch stop layer (ESL)-type TFTs. However, the BCE-type TFTs often suffer from the unstable TFT operation against the transfer conditions and the various stress conditions due to the back channel damages to the oxide semiconductor [1-3]. Our study clearly demonstrates an addition of annealing process (post-annealing) that acts as recovery annealing is effective between SiNx deposition and SiOx deposition as passivation layer for the BCE-type TFTs application. The post-annealing effect to improve the stability of the BCE-type TFTs is investigated. The Oxide TFTs were fabricated on glass substrates with gate electrodes and gate insulator. The IGZO (Indium Gallium Zinc Oxide) thin film was deposited on the substrate by DC magnetron sputtering. Next, source and drain electrodes were formed by wet-etching process using a H2O2-based etchant. The passivation layer composed of double layered SiNx/SiOx was formed by PECVD, and some samples were treated with the aforementioned post-annealing at 300 ℃ in air during the passivation layer formation. It is confirmed that the stability under the negative bias thermal illumination stress (NBTIS) of the BCE-type TFTs with the post-annealing was superior to that of the BCE-type TFT without the post-annealing. In order to clarify the variation of the chemical composition of the IGZO thin film surfaces due to the post-annealing, X-ray Photoelectron Spectroscopy (XPS) was examined as well as electronic state analysis such as photoinduced transient spectroscopy (PITS) [4,5]. [1] G. Wang, Z. Song, X. Xiao, and S. Zhang; AM-FPD 2015 Symposium Digest, pp.111. [2] M. Nag, A. Bhoolokam, S. Steudel, A. Chasin, K. Myny, J. Maas, G. Groeseneken, and P. Heremans; Jpn. J. Appl. Phys, Vol. 53, No. 11, 111401 (2014). [3] R. Zhan, C. Dong, P.-T. Liu, and an H.-P. D. Shieh; Microelectronics Reliability, 53, pp.1879 (2013). [4] J. C. Balland, J. P. Zielinger, M. Tapiero, J. G. Gross, and C. Noguet; J. Phys. D: Appl. Physics, Vol. 19, No. 1 (1986). [5] A. Hino, Y. Takanashi, H. Tao, S. Morita, M. Ochi, H. Goto, K. Hayashi, and T. Kugimiya; J. Vac. Sci. Technol. B 32 (2014) 031210. Figure 1

SUMMARY We have investigated the microwave-detected photoconductivity responses from the amorphous In–Ga–Zn–O (a-IGZO) thin films. The time constant extracted by the slope of the slow part of the reflectivity signals are correlated with TFT performances. We have evaluated the influences of the sputtering conditions on the quality of a-IGZO thin film, as well as the influences of gate insulation films and annealing conditions, by comparing the TFT characteristics with the microwave photoconductivity decay ( -PCD). It is concluded that the -PCD is a promising method for in-line process monitoring for the IGZO-TFTs fabrication.

A comparative study on light-induced negative-bias stress stabilities in amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) was performed by means of photoinduced transient spectroscopy (PITS). When the a-IGZO thin films were deposited with 4% O 2 partial pressure (P/P), a dominant peak with a maximum of around 100 K was clearly observed from the sample. There was a flow rate of SiH 4 /N 2 O of 4/100 sccm for the ESL deposition, while the PITS spectra from the sample with a flow rate of SiH 4 /N 2 O of 6/150 sccm possessed a broader peak of around 115 K and an apparent shoulder of around 200–280 K was observed. This shoulder of around the 200–280 K was clarified when the a-IGZO thin film was deposited with an O 2 P/P of 20 % In accordance with the changes in the electronic structures in the a-IGZO thin films due to the ESL deposition, the stability of the TFTs against the negative bias thermal Illumination stress (NBTIS) was degraded; the value of the V th shift after the 2h-NBTIS test was increased from 2.5 to 6.0 V The decreasing the compensating acceptors and/or the increasing the hydrogen-related donors could be the origin of the negative V th shift during the NBTIS test.

The double-layered Ti/Si barrier metal is demonstrated for the source/drain Cu interconnections in oxide semiconductor thin-film transistors (TFTs). The transmission electromicroscopy and ion mass spectroscopy analyses revealed that the double-layered barrier structure suppresses the interfacial reaction and the interdiffusion at the interface after thermal annealing at 350°C. The underlying Si layer was found to be very useful for the etch stopper during wet etching for the Cu/Ti layers. The oxide TFTs with a double-layered Ti/Si barrier metal possess excellent TFT characteristics. It is concluded that the present barrier structure facilitates the back-channel-etch-type TFT process in the mass production line, where the four- or five-mask process is used.

Electron traps in the channel region of amorphous In–Ga–Zn–O (a-IGZO) thin film transistors (TFTs) were evaluated using capacitance transient responses from thin (50 nm) metal-oxide-semiconductor diodes. It was found that the carrier transport was mainly dominated by the conduction band tail states. The trap density of a-IGZO deposited at 5 mTorr was higher than that at 1 mTorr, providing direct evidence that the trap density in the channel layer decreased by decreasing the sputtering pressure during the IGZO deposition. The defect states with activation energies of ∼0.2 eV were observed only a-IGZO deposited at 5 mTorr. The reduction of the corresponding TFT mobility should be originated from the increase in the conduction band tail states. The high pressure sputtering is suggested to enhance the structural fluctuation in a-IGZO films, at least in the structure surrounding defect, resulting in an increase of the concentration of the electron traps.

Fabrication process conditions of a passivation (PV) layer correlated with stress stabilities of amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). In etch-stop layer (ESL)-TFTs, by inserting a Si-based resin between SiN x and SiO x PV layers, the peak intensity in the photoinduced transient spectroscopy (PITS) spectrum was notably reduced. This suggested the suppression of hydrogen incorporation into a-IGZO, which led to the improvement of stability under negative bias thermal illumination stress (NBTIS). In contrast, the hydrogen-related defects in the a-IGZO were easily formed by the back-channel etch (BCE) process. Furthermore, it was found that, under NBTIS, the transfer curves of the BCE-TFTs shifted in parallel owing to the positive fixed charge located in the back channel of the a-IGZO TFTs. The hump-shaped shift increased with stress time. This is because hydrogen atoms located at the back-channel surfaces of the a-IGZO and/or PV layers were incorporated into the channel region of the BCE-TFTs and induced the hydrogen-related defects.

Negative bias thermal illumination stress (NBTIS) stabilities in amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) were studied by photoinduced transient spectroscopy (PITS). The degradation of TFT performance correlated with trap states in the channel region of a-IGZO TFTs with an etch stop layer (ESL). A prominent peak at approximately 100 K was observed in a-IGZO formed under a partial pressure (p/p) of 4% O2. With increasing O2 p/p, an apparent shoulder of around 230 K appeared in PITS spectra. A higher flow rate of SiH4/N2O for the ESL deposition induced trap states associated with the 230 K peak. The peak at approximately 100 K could originate from the depletion of Zn by preannealing, while the peak at approximately 230 K should be attributed to the oxygen-deficient and/or Zn-rich defects due to the formation of OH in a-IGZO. The trap states in a-IGZO TFTs gave rise to degradation in terms of NBTIS. The threshold voltage shift (ΔV th) was 2.5 V, but it increased with the O2 p/p as well as the flow rate of SiH4/N2O for ESL deposition. The time dependence of ΔV th suggested that hydrogen from the ESL and/or in the a-IGZO thin films was incorporated and modified the trap states in the channel region of the a-IGZO TFTs.

High-resolution Rutherford backscattering spectrometry (HRBS) in combination with grazing angle argon sputtering was carried out to characterize the interface of aluminum-nickel (Al-Ni) alloy and amorphous-silicon films in a thin film transistor (TFT) for liquid crystal display (LCD). After thinning the top Al-Ni layer by a 1-keV Ar sputtering, the sensitivity of the interface oxygen was improved to be twice higher than that before sputtering. The results revealed that the oxygen at the interface relates to the contact characteristics.

Photoinduced transient spectroscopy (PITS) was applied to study the effects of thermal annealing in the thin-film transistor (TFT) fabrication process on the variations of the electron traps in the channel region of amorphous In-Ga-Zn-O (a-IGZO). A dominant peak with a maximum of around 130 K was observed in the PITS spectra, but the detailed features were varied depending on the annealing conditions. The six particular temperatures corresponding to the trap states were extracted at about 100, 140, 150, 210, 320, and 390 K from the differential PITS spectra, showing good correlation with the trap states observed in ZnO. The results of thermal desorption spectrometry suggested that the variation of electron traps in the a-IGZO thin films has its origin in the decomposition of O and Zn during the annealing process. The annealing after the etch-stop layer deposition was also examined. The peak at about 150 K extracted from the differential PITS spectra before and after the annealing was markedly decreased. The activation energy of the corresponding trap states was estimated to be around 0.3 eV, which was close to those known as the E3 center in ZnO. Secondary ion mass spectroscopy analysis suggested that the reduction of trap density was mainly due to a decrease in the number of defects which involve hydrogen atoms in their configuration. Considering these results, the variations in the electron traps in the a-IGZO thin films during the TFT fabrication process should be attributed to the introduction of Zn, O, and/or H-related defects into tetrahedra consisting of Zn-O bonds.

The physical properties of amorphous In–Ga–Zn–O (a-IGZO) films deposited by DC sputtering under various sputtering pressures were investigated. The sputtering pressure was found to influence various physical properties. Lower sputtering pressures resulted in film densification and decreased both surface roughness and hydrogen concentration. In addition, transistor performance characteristics such as saturation mobility and sub-threshold swing improved as the sputtering pressure decreased. These results yield insight into the correlation between thin film transistor (TFT) performance and deposition conditions.

Metal-oxide-semiconductor (MOS) diodes with various gate insulators (G/Is) were characterized by capacitance–voltage characteristics and isothermal capacitance transient spectroscopy (ICTS) to evaluate the effect of H and OH desorption and diffusion on the electronic structures in amorphous In–Ga–Zn–O (a-IGZO) thin films. The density and the distribution of the space charge were found to be varied depending on the nature of the G/I. In the case of thermally grown SiO2 (thermal SiO2) G/Is, a high space-charge region was observed near the a-IGZO and G/I interface. After thermal annealing, the space-charge density in the deeper region of the film decreased, whereas remained unchanged near the interface region. The ICTS spectra obtained from the MOS diodes with the thermal SiO2 G/Is consisted of two broad peaks at around 5 × 10−4 and 3 × 10−2 s before annealing, while one broad peak was observed at around 1 × 10−4 s at the interface and at around 1 × 10−3 s in the bulk after annealing. Further, the trap densi...