**3.2. Experimental procedure**

breaking of the Si–Si bond. Here, the hydrogenation of plane poly‐Si thin films on glass with

**Figure 8.** SEM images of CLC poly‐Si after chemical etching for 25 s (a) as‐crystallized and (b) hydrogenated films.

The plasma hydrogenation process introduced as much as 1 at.% H into the ELC film. Although hydrogenation drastically improves the Hall effect mobility, excessive hydrogenation tends to degrade it. The catalytic method is useful for preventing excessive hydrogenation and damage

The H‐termination of dangling bonds at the grain boundaries can be observed indirectly or directly by chemical etching and Raman microscopy. Although preferential etching was found at the grain boundaries, hydrogenation interfered with the process because of the electro‐ chemical inactivation of dangling bond. This H‐termination appeared as 2000 cm‐1 LVM in the Raman spectra. The breaking of Si–Si bonds by hydrogenation was determined from the appearance of the 2100 cm‐1 LVM. In addition, the defects generated in the plasma process exhibited multiple fine LVMs after hydrogenation. The detection of extremely weak LVMs was caused by the enhancement of Raman scattering induced by the high‐density hillocks that are

In the case of CLC, almost no LVM was detected because of the low defect density and relatively smooth surface. Although flow‐shaped grain boundaries were revealed by the chemical etching, they were protected from the etching by hydrogenation as well as ELC poly‐Si. The density of defects residing in the grains was estimated to be considerably smaller than that observed for ELC, which was due to the unidirectional solidification and low cooling velocity

To achieve effective passivation of electrically active defects in LT poly‐Si TFTs by H, it is necessary to evaluate the interaction between H and the defects such as grain boundaries, in‐

H\* generated by plasma or catalyzer was described.

92 New Advances in Hydrogenation Processes - Fundamentals and Applications

caused by the electric‐field acceleration of charged particle.

the characteristics of ELC poly‐Si.

**3. Hydrogenation of poly‐Si TFT**

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

during recrystallization.

A self‐aligned metal double‐gate (MeDG) CLC LT poly‐Si TFT was used to detect the sensitivity variation in the performance of LT poly‐Si TFTs in order to evaluate the interaction between H and the performance of poly‐Si TFTs. **Figure 9(a)** and **(b)** shows the three‐dimensional image and top view photograph of the TFT, and **Figure 9(c)** shows the transfer characteristic and mobility of the self‐aligned MeDG CLC LT poly‐Si TFTs [40].

**Figure 9.** (a, b) Three‐dimensional image and top view photograph of TFT and (c) transfer characteristic and mobility of the self‐aligned MeDG CLC LT poly‐Si TFT.

We used only one self‐aligned MeDG CLC LT poly‐Si TFT for all the evaluations in this experiment to avoid variations in the performance of poly‐Si TFT. We used p‐channel (p‐ch) TFTs to prevent the degradation of the electrical properties of TFT under DC bias stress [41, 42]. The hydrogenation process of H2 gas annealing (N2:H2 = 97:3) was applied at 400°C for 3

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

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 technique.

#### **3.3. TFT performance**

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

**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.

The annealing procedure in **Figure 11(a)** is applied to evaluate the temperature at which the degradation of the TFT's performance begins. After hydrogenation by the stepped slow cooling, the samples were annealed in N2 gas for 60 min and the TFT performance was eval‐ uated. The red and blue lines in **Figure 11(b)** show the variation in the field‐effect mobility and subthreshold awing (SS) value at each step. The degradation of the SS value and mobili‐ ty was clearly observed after annealing at 400°C in N2 gas.

**Figure 12** shows the variations in the Si–H LVMs after W hot‐wire hydrogenation followed by annealing in N2 gas. In this experiment, hydrogenation was initially conducted under the same conditions for four samples after which N2 annealing was performed at different temperatures for each sample. This shows that the intensities of the Si–H LVMs are reduced by N2 annealing above 350°C. The same trends were observed for Si–H2 LVM.
