Amorphous InGaZnO (a-IGZO) based thin-film transistor is one of the promising candidates for stackable channel materials for next-generation memory cell devices due to its extremely low off-current (I_off) and high electron mobility. However, currently there is no active research of IGZO devices processing at thermal budgets above 550℃ and with hydrogen-rich process as a-IGZO has been reported its instability issues due to hydrogen-related defects in such processes. Therefore, we demonstrated c-IGZO thin film transistors (TFTs), which are expected to be more thermally stable under such processes, and compared their characteristics with a-IGZO TFTs.

In fig.1, agglomeration in a-IGZO was observed as a result to several high thermal hydrogen-rich deposition processes while c-IGZO remained stable without structural changes. This suggests that c-IGZO is much more immune to hydrogen at high temperature than a-IGZO and enables additional T_ox scaling. Besides, threshold voltage (V_th) can be controlled by adjusting the composition of c-IGZO (A to D), and ×1.8 higher I_on was obtained in c-IGZO (C), which has a similar V_th to that of a-IGZO (A') (Fig. 2).

Extremely low I_off of 1.82×10⁻¹⁸ A/μm was shown in the c-IGZO TFT with a channel length (L_g) of 70 nm (Fig. 3). This suggests that c-IGZO can be feasible for DRAM cell with the benefit of long retention time. 

Through composition control and T_ox scaling, Subthreshold Swing and I_on of the optimized c-IGZO show significant improvement (Fig. 4). And also, despite the relatively thin T_ox of 50 Å, the optimized c-IGZO device showed similar ΔV_th with a-IGZO (+19 mV) after positive bias temperature stress (PBTS), indicating that c-IGZO has better V_th stability (Fig. 5).

In summary, c-IGZO has better thermal stability and process immunity than a-IGZO, which resulting in improved performance as well as V_th stability over a-IGZO. Because of these characteristics, c-IGZO can be an excellent candidate of new channel materials for future memory devices which have high thermal budgets.

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