High performance polysilicon thin film transistors by H2O plasma hydrogenation

Thin Solid Films 305 (1997) 327-329

ELSEVIER

High performance polysilicon thin film transistors by H20 plasma hydrogenation Kan Yuan Lee a, Yean Kuen Fang a, Chii Wen Chen a, Mong Song Liang b, Sou Gow Wuu b a VLSI Technology Laboratory, Department of Electrical Engineering, National Cheng Kuang University, Tainan, Taiwan b Taiwan Semiconductor Manufacturing Company, Hsin-Chu, City, Taiwan

Received 17 October 1996; accepted 23 January 1997

Abstract In this work, we report an efficient way to improve characteristics of polysilicon thin film transistors (TFTs). A TFT treated with H,O microwave plasma shows an excellent subthreshold swing of 130 mV DEC-t and a threshold voltage of 2.2 V while TFT treated with conventional H 2 radio frequency plasma shows an subthreshold swing of 200 mV DE c . ~ and a threshold voltage of 3.2 V. We also find that a TFT treated with H:O plasma also shows better interface strength under electric stress than a TFF treated with H 2 plasma. The oxide integrity of a TFT with H20 plasma under constant current stress is also better than a TFT with H 2 plasma. © 1997 Elsevier Science S.A. Ke)words: Amorphous materials; Plasma processing and deposition; Silicon; Water

1. I n t r o d u c t i o n Polycrystalline silicon thin film transistors grown by low-pressure chemical vapor deposition (LPCVD) have been intensively investigated for liquid crystal displays and high density SRAM. However, due to the high trap state density at the grain boundaries in the film, a polysilicon thin film transistor (TFT) shows a larger threshold voltage and worse subthreshold swing. Hydrogenation has been reported to be an efficient method to passivate these trap states [1,2]. However, with H 2 plasma hydrogenation, the passivation effect would saturate [3]. In this work, for the first time, we find that H 2 0 plasma treatment with a microwave plasma source can also greatly improve the characteristics of a polysilicon TFT, and even shows a better passivation effect than conventional H 2 plasma treatment with radio frequency plasma source. We also discuss the degradation on the characteristics of TFTs under electric stress with H 2 0 or H 2 plasma treatments.

5 X 10 I5 c m - : and an energy of 40 keV to be the gate electrode of the TFT. Then, the gate geometry was defined and etched. The following step was a 30 nm gate oxide deposited by LPCVD at 790 °C . Next, another 30 nm amorphous silicon was deposited at 480 °C by LPCVD. Then, the channel was implanted with phosphorus at a dose of 5 × 10 ~2 cm -a and an energy of 20 keV. Then, it was recrystallized at 600 °C in N; ambient for 10 h. Source and drain regions were implanted with boron at a dose of 5 × 10 I5 cm -2 and an energy of 20 keV. Subsequent to this S / D implantation, a 400 nm of BPTEOS layer was deposited and flowed. After that, metal was deposited and patterned to be interconnect and contact pads. Some of the TFTs were covered under metal lines and some were not. The channel the of TFT is 1.2 Ixm long and 0.6 txm wide, with a 4.0 ~ m offset structure at the drain side to suppress the effect of gate induced drain

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2. Device fabrication and experiments As shown in Fig. I, the TFT used in this study is a bottom gated TFT. A polysilicon layer of 55 nm was deposited and followed by a arsenic implant at a dose of 0040-6090/97/$i7.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0040-6090(97)00055-2

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K.Y. Lee et al. / Thin Solid Films 305 (1997) 327-329

328

leakage (GIDL). H a plasma treatment was performed with a conventional plasma-enhanced chemical vapor deposition (PECVD) system at 250 °C in pure H a ambient at 1 Torr and a r.f. power of 100 W, while H , O plasma treatment was performed in a wet atmosphere ambient at a pressure of 2 Torr and with a microwave plasma source at t00W.

3. Results and discussions Fig. 2 shows the ld-Vg characteristics of TFTs treated with H 2 0 plasma for 0, 6 min, and 1 h, respectively. After 6 rain of H 2 0 plasma treatment, the performance of TFT is greatly improved. The subthreshold swing (S) is improved from 980 mV DEC-1 to 540 mV DEC-~ and the threshold voltage is from 6.4 to 3.2 V. For the longer H 2 0 plasma processing time (1 h), the characteristics of the TFT can still be improved. It shows a excellent subthreshold swing of 130 mV DEC-1 and a threshold voltage of 1.35 V. Fig. 3 shows the Id--Vg characteristics of TFTs with conventional H2 plasma treatment for 0, 1, and 4 h, respectively. We find that after 4 h of plasma hydrogenation, the performance of TFT is optimal. Comparing the optimal characteristics of the H 2 plasma treated TFT with those of the H 2 0 plasma treated T F r , we find that the H 2 0 plasma treated TFT shows better subthreshold swing (130 mV DEC - I to 200 mV D E C - l ) , off state leakage current (0.9 pA to 0.35 pA) and threshold voltage (1.7 V to 1.35 V) than those of H 2 plasma treated TFT. It has been reported that a TFT annealed in wet atmosphere shows better performance than that of a TFT without such an anneal [5]. In order to find whether the extra improvement of the H 2 0 plasma treated TFT is due to the annealing effect of HaO steam, we annealed the TFT in a wet atmosphere at 250 °C for 4 h. After that, however, we did not observe any improvement in the characteristics. We can therefore rule out the annealing effect of H 2 0 steam 16 s

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on the improvement of H 2 0 plasma treated TFI'. It may be due to the fact that the TFI" is buried under a 500 nm of PETEOS and the quantity of H 2 0 moleculars that can diffuse to the TFT device must not be high. We suggest that H 2 0 molecules can dissolve into hydrogen and oxygen ions during plasma treatment. These oxygen ions can passivate trap states by themselves a n d / o r they can enhance the passivation effect of hydrogen atoms [4]. Thus the passivation effect of H 2 0 plasma treatment is better than that of conventional H 2 plasma treatment. The other reason for the extra improvement of H 2 0 plasma treated TFT is that plasma treatment with a microwave plasma source shows lower vacuum ultraviolet emission and hence fewer interface states are generated during plasma treatment [6]. The better characteristics of a TF'f with H : O microwave plasma is due to fewer numbers of interface states being generated during plasma treatment. Fig. 4 shows the deterioration of the subthreshold swing as a function of stress time. The stress condition is Vd~= 0 V and Vg~= - 15 V. In this figure, the subthreshold swing of TFT with H a plasma treatment deteriorates more rapidly than that of TFT with H 2 0 plasma treatment. It is reported

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K.Y. Lee et al. ,/Thin Solid Films 305 (1997) 327-329

plasma. This is because the microwave plasma treatment would cause less electrical damage in the oxide than r.f. plasma treatment [6].

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that the S i - H bonds can be easily broken under electric stress [7]. Hence, due to these generated unsatisfied Si bonds, the subthreshold swing is deteriorated because more trap states have to be charged before a weak conduction channel is formed. For TFY with H 2 0 plasma treatment, in addition to hydrogen ions, there are also some oxygen ions which diffuse to the channel of TNF and form S i - O bonds. These S i - O bonds are stronger than S i - H bonds and hence they are more difficult to break than S i - H bonds under electric stress. The gate oxide integrity of both the above TNFs are examined by stressing the gate with constant current. The applied gate voltage shifts of both TF-fs under constant current stress are shown in Fig. 5. It is observed that the gate voltage shift of the TFT with conventional H 2 plasma increases more rapidly than the TFT with H 2 0 plasma and the time to destructive breakdown is also shorter for the TFT with H 2 plasma. The gate voltage shifts increase with stress time, it is due to generated traps by electrons with energy higher than 2 eV [8]. Some electrons are captured by these generated traps and reduce the electric field at the cathode (poly gate/oxide interface) which determines the magnitude of the injected current. Thus a larger negative gate stressing voltage is required to keep a constant current. This negative charging continues to increase until destructive breakdown occurs. Due to the above discussion, we can deduce that the gate oxide integrity of TFT with H 2 0 plasma is better than that of TFT with H 2

In conclusion, we find an alternative way to further improve the characteristics and reliability of polysilicon TNF. H : O plasma treatment with a microwave plasma source can improve the characteristics of a polysilicon TFT more than conventional H 2 plasma treatment with a r.f. plasma source. The interface of the H : O plasma treated TFT is also robuster than the H 2 plasma treated TFT. We suggest that this is due to the stronger bonding energy of S i - O bonds than S i - H bonds and hence the S i - O bonds are more difficult to be broken under electric stress than S i - H bonds. Moreover, by using a microwave plasma source, we can reduce the radiation damage of vacuum UV on the gate oxide of TFT during plasma treatment.

Acknowledgements The authors wish to thank Mr J.Y. Shih for his help in the H 2 0 plasma experiments~ This work was supported by the National Science Council of the Republic of China through contract NSC-85-2215-E-006-018.

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