We fabricated three kinds of poly-crystalline silicon thin film transistor Silicon-Oxide-Nitride-Oxide-Silicon (Poly-Si TFT SONOS) memory devices with a FinFET structure, an omega gate structure, and a GAA structure, respectively. The corner effect of those three structures is discussed from the device simulation, program efficiency, and the subthreshold swing shift.
2-4-1. Simulation Result
In order to demonstrate the electric field distribution of the omega gate structure and the GAA structure, a numerical simulation was carried using ISE-TCAD. Figure 2-13 and figure 2-14 show the electric field distribution of those two structures at a gate bias of 18 V. It is observed that electric field of tunneling layer in the corner is higher than that in the planar part, and electric field of blocking layer in the corner is lower than that in the planar part. Due to fabrication always has some discrepancy, the shape of the structures are not standard quarter circles. Therefore, the radius of curvature for each corner is not the same. Figure 2-15 and figure 2-16 show the distribution of electric field across the ONO stack for the two corners in the GAA structure. Since the first corner is sharper than second corner, the first corner exhibits higher electric field in the tunneling layer and lower electric field in the blocking layer.
It proves that sharp geometric structures make electric field concentrate on tunneling layer.
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2-4-2. Program efficiencies of the three structures
The Fowler-Nordheim (FN) tunneling mechanism is employed in this thesis for the FinFET structure, the omega gate structure, and the GAA structure. Figure 2-17, figure 2-18, and figure 2-19 show the transfer characteristics of the FinFET structure, the omega gate structure, and the GAA structure memory devices with various programming times at VGS= 18 V, respectively. Figure 2-20 shows the comparison of threshold voltage shifts between those memory devices after programming operation.
The FinFET structure SONOS memory device exhibits a Vth shift of 1.57 V in 10 ms, the omega gate structure exhibits a Vth shift of 2.98 V and the GAA structure exhibits a Vth shift of 3.52 V. It is obvious that the GAA structure device has highest program efficiency, the omega gate structure device comes second, and the FinFET structure device is the third.
According to the simulation results, it is quite clear that the electric field across the tunneling layer can be enhanced owing to the sharp corner geometry. Higher electric field in the tunneling layer increases the probability of carriers injecting into the charge trapping layer, and lower electric field in the blocking layer suppresses the back tunneling effect. Therefore, the program efficiency would be enhanced greatly.
A GAA structure has most corners and that is the reason why the SONOS memory device with a GAA structure exhibits highest program efficiency.
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2-4-3. Subthreshold swing shifts of the three structures
In our experiments, subthreshold swing would alter owing to two possible reasons, local charge effect and interface states.
At first, since the program speed in the corner region is faster than that in the planar part, the amount of nitride trapped charges in the corner are more than that in the planar part in a specific programming time. The electrical characteristics in the corner and in the planar part will be different, and that is called local charge effect.
More nitride trapped charges in the corner would raise the subthreshold swing shift.
Therefore, we can realize program efficiency by observing subthreshold swing shift.
Figure 2-21 shows the comparison of subthreshold swing shifts between the SONOS memory devices with a FinFET structure, an omega gate structure, and a GAA structure. The subthreshold swing shift of the SONOS memory device with a GAA structure is the largest, the shift of the SONOS memory device with an omega gate structure comes second, and the shift of the SONOS memory device with a FinFET structure is the least. The largest subthreshold swing shift means most charges inject into nitride trapping layer in the corners. Therefore, the SONOS memory device with a GAA structure has the most charges injection in the nitride trapping layer, and it exhibits the highest program efficiency at the same programming time.
On the other hand, we have already known that electric field of tunneling layer in the corner is higher than that in the planar part. Higher electric field in the tunneling layer increases the probability of carriers injecting into the charge trapping layer, but also increases the probability of damaging the channel/dielectric interface at the same time. More dangling bonds and interface traps will raise the subthreshold swing shift.
Therefore, the SONOS memory device with a GAA structure seems to have the most
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dangling bonds and interface traps, and exhibits the largest subthreshold swing shift.
2-5. Summary
We have demonstrated three kinds of poly-crystalline silicon thin film transistor Silicon-Oxide-Nitride-Oxide-Silicon (Poly-Si TFT SONOS) memory devices with a FinFET structure, an omega gate structure, and a GAA structure, respectively. From simulation results, the sharp geometry of corner enhances the electric field at the Si/tunneling oxide interface and depresses the electric field in the blocking oxide.
Higher electric field in the tunneling layer and lower electric field in the blocking layer enhance the program speed greatly. Moreover, a GAA structure has most corners. Consequently, the SONOS memory device with a GAA structure exhibits highest program efficiency.
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