Material Characterization of SA-BG LTPS Thin Films

3.3.1.1 Optical Microscopy (OM) Analysis

Figure 3-3 (a) and 3-3 (b) shows the optical microscope (OM) images of the samples with bottom gate structure from the transparent light source. The thickness of the amorphous silicon gate electrode was both 1000Å and the channel length was 2 µm and 5 µm, respectively. At first, it was confirmed that the bottom-gate maintained its original structure after excimer laser crystallization. In addition, the bottom amorphous silicon gate is thick

(100 nm) enough to act as a mask for the formation of the self-aligned bottom gate structure by using the back surface exposure. According to those pictures, the regions above the amorphous silicon bottom-gate were dark but the other regions were bright in both cases. It could be found that the g-line light could not pass through the region sheltered by the amorphous silicon (a-Si) bottom-gate but could pass through other regions not covered by bottom-gate electrode. The g-line light was mostly absorbed and reflected by the amorphous silicon bottom-gate electrode. Hence, the thicker a-Si bottom gate, the less g-line light crossed the bottom-gate. Therefore, the a-Si bottom-gates with thickness of 1000 Å and 1500 Å could act as the opaque masks of the photo-lithography process to stop the ultra violate light from the Hg light source. Figure 3-4 (a) and 3-4 (b) shows OM images of the mis-aligned and the self-aligned ion-implanted devices after photo-lithography from the reflected light source, in which the channel length was 2 µm. Due to the process of masker aligner, there were horizontal shifts of photo-resist (P.R.) on the region of a-Si bottom-gate in defining the source/drain regions which result in the offset region after the photo-lithography, as shown in the Figure 3-4 (a). The horizontal shift was about 0.45 µm which would lead to the mis-aligned process of the ion implantation and degrade the device performance. Figure 3-4 (b) show a self-aligned photo-lithography by backside exposure method. It could be observed that a photo-resist pattern designed on the 100 nm-thick a-Si bottom-gate after the backside exposure photo-lithography, as shown in Figure 3-4 (b). The bottom gate pattern was copied for the photo-resist coating on the bottom-gate. Owing to the perfect self-aligned back surface exposure, the P.R. on the bottom-gate would not absorb UV light. Therefore, the P.R. would be remained and perfectly aligned to the bottom-gate after the develop process. Consequently, the self-aligned ion implantation of source and drain regions to gate would be precisely carried out without any shifts.

Figure 3-5 shows the SEM micrograph of the self-aligned ion-implanted devices after the photo-lithography. According to the SEM micrograph, the P.R. with thickness of 1.2 µm was observed and the P.R. was perfectly aligned to the a-Si bottom-gate electrode which was consistent with the results obtained by OM graphs. To sum up, the a-Si bottom-gate could act as the opaque mask of the photo-lithography process to stop the ultra violate light from the Hg light source and this method leaded to the easy formation of a self-aligned bottom-gate TFTs.

As a result, the P.R. aligned to the amorphous silicon gate would benefit the ion implantation of source and drain regions for the minimal series resistance.

Figure 3-6 (a) and 3-6 (b) display the SEM photographs of excimer laser crystallized poly-Si with bottom-gate structure after Secco etching, in which the thickness of bottom-gate electrode was 100 nm, and 150 nm, respectively. In these cases, the length of bottom-gate is 1.5 µm, the laser energy density is 450 mJ/cm² and the substrate temperature is maintained at room temperature during laser irradiation. According to the SEM graphs in the Fig. 3-6(a) and 3-6(b), it can be observed that the large silicon grains with 0.75 µm in lateral dimension could be formed in the channel regions, while small and fine grains were located near the edges of the bottom-gate for the 100 nm-thick and 150 nm-thick bottom-gate electrodes, respectively.

3.3.1.3 Transmission Electron Microscopy (TEM) Analysis

The micro-structural properties of laser-crystallized poly-Si films with bottom-gate structure such as the grain size, inter-grain defect density, intra-grain defect density, and grain orientation can be identified by using transmission electron microscopy (TEM) and its selected-area electron diffraction patterns. Figure 3-7(a) displays the cross-sectional TEM image and the selected-area electron diffraction patterns of 1000 Å-thick poly-Si thin films with bottom-gate structure after laser crystallization, in which the device channel length is 1.5

µm. In this case, the laser shot number is 100 shots and the bottom-gate thickness is 1000 Å.

The laser energy density is 450 mJ/cm² and the substrate temperature is maintained at room temperature during laser irradiation. It is observed that four large silicon grains with good crystallinity are formed based on the ELC with bottom-gate structures. It is further confirmed that the lateral grain growth using ELC with bottom-gate structure can also be fabricated on the quartz wafer. For the short channel length with the proper thicknesses of gate electrode, gate oxide, and channel layers using this crystallization, only single grain boundary perpendicular to the channel direction is also observed by TEM image, as shown in Figure 3-7(b). According to the TEM image in the Figure 3-7 (b), there are two large silicon grains formed in the channel region above the bottom-gate electrode and the high angle grain boundary can be artificially controlled in the middle of the channel region. The crystallinity of the silicon grain in the channel region is pretty good and the normal orientation of the grain is in <110> direction due to the simple spots based on the selected-area electron diffraction patterns. In addition, the cross-sectional TEM image in Figure 3-7 display the clear interfaces between the bottom-gate oxide and poly-Si active layer. The interface between the bottom-gate electrode and bottom-gate oxide is also clear, implying that both the gate oxide and the bottom-gate electrode are not damaged during excimer laser irradiation. Moreover, the cross-sectional TEM image reveals that a flat interface morphology between the gate dielectric and poly-Si channel films in the bottom-gate TFTs. The smoother interface of bottom-gate TFT implies that the proposed TFT will exhibit improved breakdown characteristics and better reliability [3.30]-[3.31]. The performance and uniformity of TFT devices can be improved with such artificially large laterally grains. In addition, the circuit layout design is easier because proposed crystallization method is insensitive to laser scanning direction or device location.