I. INTRODUCTION

Amorphous In–Ga–Zn–O (a-IGZO) has been widely used as a material for channel layers of transparent oxide semiconductor (TOS) thin-film transistors (TFTs) because of its high field-effect mobility of more than 10 cm²/V·s, low process cost, and low process temperature on glass or flexible substrate^[1-3].

In the fabrication of a-IGZO TFTs, a-IGZO thin films are generally deposited by sputtering and annealed using a conventional furnace such as an oven, vacuum furnace, or rapid thermal processing (RTP). Although the conventional annealing process may be carried out at a relatively low temperature, it usually takes a long time to complete the annealing process considering the convection in the oven or the heating and cooling process in the vacuum furnace or RTP equipment.

For a reduction in the process time, xenon (Xe) flash lamps using arc discharge have been tried for the annealing process in the fabrication of hydrogen degenerated amorphous silicon (a-Si:H) thin films instead of excimer laser annealing, because of its low process temperature and adaptability to the in-line continuous process^[4-7]. Because Xe flash lamp annealing can be achieved over large areas of the samples, it is possible to obtain more improved uniformity compared with excimer laser annealing.

Xe flash lamps are operated within a short time of less than 1 s, as shown in Fig. 1. Compared with other conventional continuous annealing processes, it is possible to use the high peak power in the annealing process during pulse operation, reducing the process time. A Xe flash lamp has effective emission spectral characteristics in the wavelength range of ultraviolet (UV) and visible rays.

Fig. 1. Comparisons of the pulsed operation of Xe flash lamp annealing and the continuous operation of conventional annealing process.

Considering its pulse operation and optical characteristics, it is possible to anneal thin films selectively, thus reducing the damage on the substrate. Recently, flash lamp annealing has been applied to various semiconductor materials^[8] and transparent conductive oxide films, such as indium tin oxide^[9]. Some oxide TFTs were reported that used a flash lamp annealing process^[10]. In this experiment, Xe flash lamp annealing was applied to a-IGZO semiconductor films on different substrates and the fabrication of an a-IGZO TFT on Si wafer. From the electrical characteristics of the annealed a-IGZO thin films and a-IGZO TFTs, the annealing effect of the Xe-flash lamp was investigated and compared with the conventional annealing process.

II. EXPERIMENTAL DETAILS

Soda-lime glass, polyethylene terephthalate (PET), and a p-type Si wafer with a thermally grown SiO₂ layer were used as substrates. A 30-nm-thick a-IGZO active pattern was formed by using a radio-frequency (RF) sputtering method through a shadow mask pattern. During RF sputtering, the room temperature was maintained, and the power was kept at 60 W with a pressure of 4 mTorr and an Ar flow rate of 50 sccm. The IGZO thin films on glass, PET, and Si wafer were loaded on a Xe flash lamp system (PSTEK, Flash lamp power supply), and the annealing process was carried out at a voltage of 300 V, as shown in Fig. 2(a). The distance between the IGZO sample and Xe flash lamp was 1 cm.

Fig. 2. Schematic diagram of (a) xenon flash lamp, (b) vacuum furnace for a-IGZO annealing.

The pulse of the Xe flash lamp flash was applied for various repetition numbers from 5 to 20 with a pulse width of 15 ms. The power density of the Xe flash lamp was 6.39 J/cm² in the above conditions with a power rise of 1000 W/s. For the comparison of the annealing effects, other IGZO thin films on various substrates were put into the vacuum furnace, as shown in Fig. 2(b). After the vacuum was pumped to 5 × 10^-5 Torr, the temperature was increased to 150℃, and the annealing process was performed while maintaining 150℃ for 5 min. The annealed IGZO thin films were electrically characterized by Hall measurement systems (ECOPIA, HMS-3000R), and the results were compared with those for thin films that were not annealed. For the fabrication of a-IGZO TFTs, the heavily p-type doped Si wafer was also used as a gate electrode instead of metal electrodes, and the thickness of SiO₂ was about 100 nm. On the annealed a-IGZO patterns on the SiO₂/Si wafer, source/drain electrodes were formed by electron beam evaporation of titanium. The electrical characteristics were measured using a probe station and HP-4156.

III. RESULTS AND DISCUSSION

Fig. 3 shows the spectral characteristics of the Xe flash lamp used in the experiments. The spectral radiance was measured using a spectrum scanner (PR-788, Photo Research). Although multiple minute spectrum peaks were investigated, it was possible to observe a broad light spectrum mainly over the visible wavelengths instead of infra-red (IR) wavelengths.

Fig. 3. Spectral characteristics of Xe flash lamp used in the experiments.

From the Hall measurement, the annealing effects of the Xe flash lamp on a-IGZO thin films were investigated. Fig. 4 shows the carrier mobilities of the annealed a-IGZO thin films on various substrates for different annealing process conditions. In the case of nonannealed a-IGZO, mobilities of approximately 6.98, 12.29, and 29 cm²/V·s were obtained for soda-lime glass, PET, and SiO₂ on Si wafer, respectively. For a-IGZO thin films annealed at 150℃ in a vacuum furnace, higher mobilities of 33.93, 88.66, and 136.7 cm²/V·s were measured for glass, PET, and SiO₂ on Si wafer, respectively. After Xe flash lamp annealing, it was possible to observe the improvements of a-IGZO mobilities for pulse repetition numbers of 10, 15, and 20. The Xe flash lamp showed almost the same annealing effect as a conventional continuous annealing process. For the pulse repetition number of 20, higher mobilities of 283.4, 87.26, and 122.7 cm²/V·s were investigated for glass, PET, and SiO₂ on Si wafer, respectively. In the case of PET substrates, the a-IGZO thin films annealed using a Xe flash lamp for the pulse repetition number of 5 also showed enhanced carrier mobility. From the spectral characteristics of the Xe flash lamp, the measured carrier mobilities of a-IGZOs on PET substrates show that the IGZO thin films on PET substrates were selectively heated and annealed, minimizing the absorption of energies of the Xe flash lamp on PET substrates^[11,12]. Considering the optical transmittance of PET, it is possible to use the Xe flash lamp for the selective annealing of transparent conductive electrodes (TCEs) or TOS on flexible substrates.

Fig. 4. Carrier mobilities of a-IGZO thin films on soda-lime glass, PET, and SiO₂ on Si wafer for different annealing conditions.

Fig. 5 and Table 1 show the comparison of transfer characteristics and extracted electrical parameters of a-IGZO TFTs fabricated on Si wafers for various annealing conditions of Fig. 4. Electrical measurements were carried out for a-IGZO TFTs with a channel width of 170 $\mu m$ and a length of 85 $\mu m$ , respectively. At a drain-to-source voltage ($V_{DS}$) of 10 V, the nonannealed a-IGZO TFT showed a saturation field-effect mobility ($\mu _{sat}$) of 2.43 cm²/V·s, a subthreshold swing (SS) of 1.08 V/decade, a threshold voltage ($V_{th}$) of -4.1 V, and an on–off ratio of 6.15 × 10⁴. For the a-IGZO TFT annealed at 150℃ in a vacuum furnace, a much higher $\mu _{sat}$ of 12.89 cm²/V·s, a steeper SS of 0.76, a higher on–off ratio of 2.07 × 10⁵, and a positively shifted $V_{th}$ of 3.71 were observed. In the case of a-IGZO TFTs annealed by a Xe flash lamp, it was possible to observe a much higher $\mu _{sat}$ of 11.72, 12.33, and 12.55 cm²/V·s and much higher on–off ratios of 2.07 × 10⁵, 2.28 × 10⁵, and 2.23 × 10⁵ for pulse repetition numbers of 5, 10, and 15, respectively. Moreover, a much steeper SS of 0.92, 0.65, and 0.87 and positively shifted $V_{th}$ of 2.55, 4.69, and 4.71 were obtained for the pulse repetition numbers of 5, 10, and 15, respectively. $\mu _{sat}$ and $V_{t}$h were obtained by the slope and the intercept of the extrapolated curve calculated from the square root of the transfer characteristics with a voltage axis. The on–off ratio was calculated from the on-current ($I_{on}$) and off-current ($I_{off}$) defined as the drain–source current at a gate–source ($V_{gs}$) voltage of 20 V and -5 V, respectively. Compared with the a-IGZO TFTs nonannealed and annealed in a vacuum chamber, the a-IGZO TFTs showed more improvement in electrical characteristics. However, in the case of a pulse repetition number of 20, a smaller $\mu _{sat}$ of 1.54, a larger SS of 1.87, a reduced on–off ratio of 4.02 × 10⁴, and some degradation in transfer characteristics were investigated. The degradations are expected to have resulted from the damage caused by excessive repetition of pulses of the Xe flash lamp.

Fig. 5. Transfer characteristics of a-IGZO TFTs for various annealing conditions.

Fig. 6 shows the output characteristics of the IGZO TFTs of Fig. 5. From the results, it is possible to conclude that the Xe flash lamp with pulse operation has an annealing effect on IGZO TFTs similar or superior to that of the conventional annealing process in continuous mode. From the results, the Xe flash lamp annealing effect is expected to be applicable to other n-type or p-type oxide thin films or TFTs. The flash lamp is expected to be one of the low-temperature annealing methods in the fabrication of electronic devices including semiconductor materials.

Fig. 6. Output characteristics of a-IGZO TFTs for various annealing conditions.

IV. CONCLUSIONS

As a new method for low-temperature and low-cost annealing processes of oxide semiconductors with a high productivity, a Xe flash lamp was used inthe annealing of a-IGZO thin films and applied to the fabrication of a-IGZO TFTs for various pulse repetition numbers. From the Hall measurements of IGZO thin films and current–voltage transfer characteristics of IGZO TFTs annealed by a Xe flash lamp, it is possible to investigate a higher mobility and on–off ratio, a steeper SS, and a threshold voltage shift in the case of annealing at higher pulse repetition numbers. The Xe flash lamp annealing process is expected to be widely applied to the fabrication of various semiconductor devices.