Crystalline IGZO Transistors With High Thermal Stability Show Promise for Next-Gen Memory Channels

Amorphous InGaZnO¹ (a-IGZO)-based thin-film transistors² (TFT) have shown potential as stackable channel materials for next-generation memory solutions due to their extremely low off-current (I_off) and high electron mobility. However, currently there is no active research being conducted on IGZO devices operating at thermal budgets³ above 550°C during hydrogen-rich processes, which are normally used in memory development. This results in a-IGZO instability issues driven by hydrogen-related defects during these processes.

¹InGaZnO: A semiconducting material consisting of indium (In), gallium (Ga), zinc (Zn), and oxygen (O).
²Thin-film transistor (TFT): A type of MOSFET fabricated through thin-film deposition traditionally used in liquid crystal displays (LCDs).
³Thermal budget: The total amount of thermal energy or heat that can be dissipated or allowed within a device without exceeding its specified temperature limits.

In light of this, SK hynix’s Revolutionary Technology Center (RTC) conducted research on crystalline IGZO (c-IGZO) TFTs and compared their characteristics with a-IGZO TFTs. Presented at the 2023 Very Large-Scale Integration (VLSI) Symposium, the study aimed to demonstrate that c-IGZO TFTs can be more thermally stable than a-IGZO under hydrogen-rich processes.

Demonstrating the Thermal Stability & Hydrogen Process Resistance of c-IGZO

Figure 1. (a) The various process steps involving hydrogen at 550°C after IGZO deposition. (b) Transmission electron microscopy (TEM) images of a-IGZO and (c) c-IGZO before and after subsequent deposition processes and electron energy loss spectroscopy (EELS) images after processing.

Figure 2. (a) The transfer characteristics of a-IGZO (A’) and c-IGZO TFTs (A-D), which have incrementally increasing amounts of gallium from A to D, at a drain voltage (V_ds) of 1V (W/L=0.8/0.1 micrometers [μm]). (b) A comparison of the on-current (drain current [I_ds] at gate voltage [V_gs]-threshold voltage [V_th]=3V) with V_th performance for various IGZO conditions.

As shown in Figures 1 (b) and (c), agglomeration⁴ was observed in a-IGZO following several high thermal hydrogen-rich deposition processes, while c-IGZO remained stable without structural changes. This suggests that c-IGZO is significantly more resistant to hydrogen at high temperatures than a-IGZO and enables additional oxide thickness (T_ox) scaling. Meanwhile, Figure 2 (b) shows that the threshold voltage (V_th) can be controlled by adjusting the composition of c-IGZO (A to D). When c-IGZO demonstrated a similar V_th to a-IGZO, the on-current (I_on) was 1.8 times higher in c-IGZO (C) than a-IGZO (A’).

⁴Agglomeration: The process of particles or granules sticking together to form larger clusters or agglomerates.

Figure 3. An off-current at 25°C of c-IGZO (C) TFT with a channel length of 70 nanometers (nm) extracted from an Arrhenius plot

Figure 4. The transfer characteristics of a-IGZO (T_ox=100Å) and optimized c-IGZO TFT (T_ox= 50Å). (V_ds = 1V)

Figure 5. The optimized c-IGZO (T_ox =50Å) TFT after positive bias temperature stress (PBTS) testing for 1,000 seconds (s).

Figure 3 shows that extremely low I_off of 1.82×10⁻¹⁸ A/micrometers (μm) was demonstrated in the c-IGZO TFT with a channel length⁵ (L_g) of 70 nanometers (nm). This suggests that c-IGZO can be a feasible material for DRAM cells as it offers a long data retention time.

The researchers also found that, through composition control and T_ox scaling, the subthreshold swing⁶ and I_on of the optimized c-IGZO showed significant improvement (Figure 4). Furthermore, Figure 5 shows that despite the relatively thin T_ox of 50Å, the optimized c-IGZO device demonstrated similar V_th shift (ΔV_th) as a-IGZO (+19 millivolts [mV]) after the positive bias temperature stress⁷ (PBTS) test. This indicates that c-IGZO has better V_th stability than a-IGZO.

⁵Channel length: A critical dimension of a MOSFET which represents the length of the semiconductor channel between the source and drain terminals.
⁶Subthreshold swing: The amount of change in the gate voltage required to change the drain current by a factor of 10.
⁷Positive bias temperature stress (PBTS): A reliability test for a semiconductor device which involves subjecting the device to elevated temperatures while applying a positive bias voltage to the gate terminal.

C-IGZO: The Future of Next-Gen Memory Channel Materials

The researchers found that c-IGZO has better thermal stability and is more immune to hydrogen processes than a-IGZO. Due to these characteristics, c-IGZO can be an excellent candidate for new channel materials in future memory devices with high thermal budgets.

For more information regarding RTC’s research, please visit the center’s research website (https://research.skhynix.com). The RTC operates the site to share insights on its ongoing research of future technologies and to actively communicate with various global research organizations.