Effects of Ni concentration on thermal stability

Also of concern is the thermal stability, which was examined at elevated temperatures.

Figure 3-11 presents the I-V curves of the MIC and CF-MIC, which were performed at temperature from 25 to 125°C. As can be seen, the off-state curves were raised with increase of operation temperature. The threshold voltage shift and off-state current as a function of temperature were summary in Fig. 3-12. As the results, the threshold voltage and the minimum leakage current of devices were degraded with increasing the operation temperature.

This is because nickel related donor-like defects were easy to release electrons when Fig. 3-10 Schematic weaker Si-H bonds and Si-Si bonds at MIC grain boundaries of plane-view.

operation temperature increased, thus increasing the leakage current and the negative shift of Vth [73], [87]. Compared with those of MIC, the thermal stability of CF-MIC was improved by introducing a chemical oxide layer, which is due to the reduction of Ni concentration in devices. Consequently, the increase of the leakage current and the negative shift of Vth of CF-MIC was less than that of MIC. In a word, it is a appropriate course to reduce Ni concentration in MIC TFTs, which shows not only better on-state reliability at bias stress but also better off-state stability at elevated temperature.

Fig. 3-11 I-V curves of the MIC and CF-MIC at temperature from 25 to 125°C .

Fig. 3-12 Degradations of the threshold voltage and the minimum leakage current versus temperature at VDS = 5 V.

3.4 Summary

It is well known that reducing Ni concentration in MIC films is an effective way to improve leakage current. This study investigated how Ni concentration affects electrical characteristics of MIC TFTs, such as S/D series resistance, bias reliability and thermal stability. For comparison, high and low Ni concentration devices were formed by using MIC TFTs with and without a chemical oxide layer, respectively.

In the part 1, we have provided further insight into how Ni concentration and resistance of MIC TFTs are related. Consequently, the channel resistance and S/D series resistance were decreased with the reduction of Ni concentration in MIC poly-Si due to better crystalline quality and lower degradation of donor concentration. This phenomenon is owing to that low Ni concentration formed less nucleation site of NiSi2 to cause large grain size; Ni atoms serve as acceptor-like dopants in silicon, which counteract the effects of n-type doping, subsequently reducing the donor concentration in the S/D region.

In the part 2, the Ni concentration effect on bias reliability and thermal stability were investigated under hot carrier stress and elevated temperature, respectively. We have proved that reducing Ni concentration in MIC films was also beneficial for bias reliability and thermal stability. As the results, the low Ni residues device (CF-MIC) presented high immunity against the hot-carrier stress because larger grain size and fewer weak Si-H bonds.

Moreover, it was also found that reducing Ni concentration can alleviate the degradations of

threshold voltage and off-state current at elevated temperatures because nickel related donor-like defects were easy to release electrons with increase of operation temperature.

In sum, results of chapter 2 and chapter 3 demonstrate that the Ni residues obstruct the performance, S/D conductivity, bias reliability and thermal stability. Consequently, these findings verified that reducing Ni residues is a significant way to improve electrical characteristics of MIC TFTs.

Chapter 4

Improved electrical characteristics and reliability of MIC TFTs Using Drive-In Nickel Induced

Crystallization

4.1 Introduction

For use in active-matrix liquid crystal displays (AMLCDs) active-matrix organic light-emitting diodes (AMOLED), low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) have attracted considerable interest because they exhibit good electrical properties and can be integrated in peripheral circuits on inexpensive glass substrates [47]-[48]. Intensive studies have been carried out to reduce the crystallization temperature and time of amorphous silicon (α-Si) films. Metal-induced crystallization (MIC) is one of these efforts. The advantages of MIC include low cost, good uniformity, low crystallization temperature (~500 ) and ℃ short crystallization time (0.5 to 5 h). Unfortunately, Ni and NiSi2

residues in the poly-Si film increased the leakage current and shifted the threshold voltage [52]-[53]. Therefore, Ni concentration in MIC poly-Si should be reduced to improve the off-state current. The atomic layer deposition (ALD) technology and gettering method have been employed to reduce the amount of undesired metal impurity. However, both methods are

complex and incur high cost [88]-[89], notably the device performance in on-state current is decreased [54].

To increase on-state current, hydrogen has been employed to eliminate the intragrain and grain boundary trap states in the poly-Si film [90]-[92]. However, the hydrogenated poly-Si TFTs suffer from a serious reliability issue, which is attributed to the weak Si-H bonds. Recent, several studies have devotedly demonstrated that introduction of fluorine (F) atoms can improve the performance and reliability of poly-Si TFTs, especially under electrical stress owning to strong Si-F bonds more stable than Si-H bonds [76], [93]-[94].

Unfortunately, the minimum off-state currents were almost unchanged, probably because the Ni concentration was constant.

This study proposes two processes for improving the performance of MIC TFTs. In the part 1, a new manufacturing method for poly-Si TFTs using drive-in Ni induced crystallization (DIC) was proposed. In DIC, F⁺ implantation was used to drive Ni in the α-Si layer. In the part 2, a chemical oxide layer was introduced between the Ni and α-Si layer.

With these two processes, the electrical performances of TFTs were both improved. Moreover, thermal stability and bias reliability of fluorinated MIC TFTs were investigated.

4.2 Part 1: Investigation of driven-in Ni induced crystallization