Deposition Technique of IGZO Film for Large Sputtering Cathode

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3.Evaluation of an IGZO film deposited by the moving cathode

3.1 Uniformity of the film thickness
Figure 8 Distribution of thickness of an IGZO film.
Figure 8 Distribution of thickness of an IGZO film.
(a) Distribution of thickness of a film on a G6 substrate.
(b) Distribution of film thickness in transverse direction.

Figure 8 presents the thickness distribution of an IGZO film deposited on a 6th-generation substrate (G6 substrate) with a dimension of 1,850 mm×1,500 mm. A high uniformity of±2.9% was demonstrated for the film on the G6 substrate. Unlike films made by stationary substrate deposition, no unevenness was caused by the cathode in the transverse direction, achieving a remarkable thickness distribution of ±1.6% (Figure 2). The evaluation demonstrated that the moving cathode is capable of depositing a fairly uniform thickness film.

 

Figure 9 Evaluation results of initial characteristics of an IGZO TFT with 25 points on a G6 substrate.
Figure 9 Evaluation results of initial characteristics of an
IGZO TFT with 25 points on a G6 substrate.
3.2 TFT characteristics

Figure 9 presents the initial characteristics of the IGZO TFT as evaluated at 25 points on the surface of a G6 substrate (1,850 mm×1,500 mm). Vgs in the figure represents the gate-source voltage whereas Ids represents the drainsource current. The mobility (μ) in the saturated region was 7.5 cm2/V・sec on average according to the calculation with sub-threshold swing marking 0.32 V/dec. Von (i.e., Vgs at Ids=9 nA) marked 1.1±0.25 V. Excellent and consistent results were obtained with the initial TFT characteristics across the surface of the substrate. Subsequently, the bias stability of the IGZO film was evaluated at 17 points on the same substrate. The results are presented in Figure 10. Vds denotes the source-drain voltage.
In terms of stress conditions, the positive-bias temperature stress (PBTS) was maintained at Vds=+20 V with the substrate temperature at 60℃, and the negative-bias illumination temperature stress (NBITS) was maintained at Vds=-20 V with the substrate temperature at 60℃, while 4,500 cd/m2 of white LED light was constantly irradiated.
Sixty minutes after PBTS was applied, Von shifted by 0.24 to 0.98 V and after NBITS was applied it shifted by -1.30 to -0.5 V. The small size of the shifts in Von indicates that the cathode can provide even bias stability on the substrate surface. Together these results demonstrate that deposition of an IGZO film with a moving cathode can achieve high uniformity and excellent bias stability.

Figure 10 Evaluation results of bias stability of an IGZO TFT with 17 points on a G6 substrate.
Figure 10 Evaluation results of bias stability of an IGZO TFT with 17 points on a G6 substrate.

 

4.Conclusion

A new kind of cathode, a moving cathode, was developed in order to achieve high uniformity and excellent bias stability in film deposition on large substrates. An IGZO film deposited on a G6 substrate (1,850 mm×1,500 mm) by this cathode marked a thickness distribution of ±2.9%. In terms of TFT characteristics, the mobility was 7.5 cm2/V・sec, the sub-threshold swing was 0.32 V/dec, and Von was 1.1±0.25 V. The evaluation using PBTS and NBITS demonstrated the high uniformity and excellent bias stability of the film. Based on these findings, the moving cathode proved capable of performing IGZO film deposition with high uniformity and excellent bias stability.

5.Acknowledgement

The authors express their deep appreciation to Professors Hideo Hosono, Toshio Kamiya, and Hideya Kumomi for their generous advice in conducting this research.

*This article was released in “Technical Journal No.79 published in June, 2015”

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Yasuo OOKUBO*1, Tastunori ISOBE*1, Makoto ARAI*1, Junya KIYOTA*1, Kazuya SAITOU*1, Tesuhiro OONO*2,
Hiroki OOZORA*2 and Shigemitsu SAITOU*2
*1 Institute for Super Materials, ULVAC, Inc., 10-2, Misawa, Tomisato, Chiba, 286-0225, Japan
*2 FPD・PV Division, ULVAC, Inc., 2500 Hagisono, Chigasaki, Kanagawa, 253-8543, Japan
We developed new cathode “Moving Cathode” in order to accomplish high uniformity and high bias stability sputtering onto a large scale substrate. As a result of sputtering amorphous Indium-gallium-zinc oxide (a-IGZO) using this cathode, there indicated the film thickness distribution of ±2.9% on Generation6 substrate (1850 mm×1500 mm) and mobility: 7.5 cm2/V/sec, Sub-threshold swing: 0.32 V/dec, Von=1.1±0.25 V at TFT characteristics. Also in bias stability both positive bias temperature stress and negative bias illumination temperature stress, it demonstrated high uniformity and high bias stability.