**3.4. Gettering of hydrogen in poly‐Si TFTs**

h in a furnace tube. The diffusion coefficient of H at 400°C is very large in Si and SiO2, and therefore, this hydrogenation condition is sufficient for the introduction of H into the TFT. The TFT was subjected to annealing in N2 gas at 450°C for 60 min before hydrogenation annealing to initialize its performance. This annealing process results in the initialization of the TFTs, as

94 New Advances in Hydrogenation Processes - Fundamentals and Applications

The behavior of H in the poly‐Si film after hydrogenation or dehydrogenation annealing is an indispensable bit of information for determining the mechanism of hydrogenation in LT poly‐ Si TFTs. The Si–H and Si–H2 LVM in the poly‐Si thin film was directly measured using an ELC poly‐Si film with a grain size of 300 nm and Raman microscopy to determine this mechanism. The hydrogenated ELC poly‐Si film was prepared by the tungsten (W) hot‐wire catalytic

The hydrogenation process in H2 gas annealing follows three different cooling processes: quenching, slow cooling, and stepped slow cooling; these are shown in **Figure 10(a)**. Quench‐ ing indicates the quick removal of the TFT substrate from the furnace tube into ambient air. **Figure 10(b)** shows the transfer characteristic of the three different cooling processes. **Fig‐ ure 10(c)** shows a summary of the TFT's performance. The field‐effect mobility was calculated in the linear region under a drain bias Vd of ‐100 mV assuming a TG structure. This indicates

**Figure 10.** (a) Hydrogenation process for three different cooling processes, (b) transfer characteristic for the three dif‐

ferent cooling processes in (a) and (c) summary of TFT performance.

that the stepped cooling process is attractive to improve performance of TFTs.

shown in **Figure 11(b)**.

**3.3. TFT performance**

technique.

The reduction in the Si–H and Si–H2 LVM above 400°C is caused by dehydrogenation. Since the ELC poly‐Si film is thin, it is easy to decrease its volumetric H concentration. This phe‐ nomenon also arises in poly‐Si TFTs. However, in this case, out‐diffusion of H starts at the surface of the SiO2 interlayer followed by the poly‐Si layer.

**Figures 11** and **12** show that 400°C is a considerably high temperature for hydrogenation because the binding energy (U) between H and the defects, namely the grain boundaries, in‐ grain point defects, interface state of poly‐Si/SiO2 and defects in dielectric in the poly‐Si TFTs, is lower than the thermal energy at 400°C. Once the H is gettered by defects, it is easily released from them at 400°C, as shown in **Figure 13(a)**. However, at temperatures below 350°C, U is greater than the thermal energy, as shown in **Figure 13(b)**. Thus, temperatures lower than 400°C are important for gettering of H in a poly‐Si TFT.

**Figure 11.** (a) Annealing procedure used to evaluate the temperature at which the degradation of TFT performance begins and (b) variation in the field‐effect mobility and SS value at each step.

**Figure 12.** Variations in the Si–H LVM after W hot‐wire hydrogenation followed by annealing in N2 gas.

**Figure 13.** (a) Interaction between defects and H above 400, (b) Interaction between defects and H below 350.

**Figure 14** explains the hydrogenation phenomenon from the perspective of gettering for two types of cooling processes after hydrogenation annealing at 400°C. As mentioned above, the thermal energy at 400°C is greater than the binding energy U between H and the defects in the poly‐Si TFTs. Therefore, at 400°C, much of the H in the poly‐Si TFTs migrates without being trapped by defects. If we used the quenching process, H diffusion motion stops be‐ cause of the rapid cooling without diffusing over a long distance. Therefore, only the H in the neighborhood of the defects is gettered, which leads to a small amount of gettered H. If we used the stepped slow cooling, the diffusion length of H becomes very large. Therefore, H that is far from the defects can combine with the defects during the cooling process, and the amount of H accumulated at the defects is greater than that in the process with quench‐ ing and slow cooling. This leads to effective passivation of electrically active defects in poly‐ Si TFTs. Thus, the rate of cooling from 400°C is the most important parameter for hydrogenation of poly‐Si TFTs by H2 gas annealing.

**Figure 14.** Hydrogenation phenomenon from the perspective of gettering for two types of cooling processes.

#### **3.5. Summary of the hydrogenation of poly‐Si TFTs**

**Figure 12.** Variations in the Si–H LVM after W hot‐wire hydrogenation followed by annealing in N2 gas.

96 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 13.** (a) Interaction between defects and H above 400, (b) Interaction between defects and H below 350.

We investigated the process of hydrogenation in terms of the gettering phenomenon in a high‐ performance p‐ch self‐aligned MeDG LT poly‐Si TFT. Hydrogenation was carried out by H2 gas annealing at 400°C for 3 h. Our results show that the hydrogenation temperature of 400°C is rather high, and at this temperature, the gettered H is re‐emitted. In the H2 gas annealing, the hydrogenation of poly‐Si TFTs actually occurs when it is cooled to temperatures below 400°C, but not at 400°C. The most important parameter is the rate of cooling from 400°C. In this experiment, the differences between the hydrogenation phenomena depending on the origin of defects were not considered. We need further investigation to clarify the effects of these differences.
