Atomic Layer Deposition of WO_x-Doped In₂O₃ for High-Performance BEOL-Compatible Transistors

The semiconductor industry has undergone a revolution over the last five decades, driven by Moore’s statement on dimensional scaling. However, the challenges for continued area scaling via lithographic patterning and the memory bottleneck limit the efficiency of computing. A new paradigm of improved energy efficiency, high bandwidth, and low latency on-chip memory access can be achieved by using 3D computing architectures with monolithic integration of logic and memory devices at high density (M3D, Monolithic three-dimensional). To realize M3D integration in future computing, fabricating high-performance thin film transistors and memories under back-end-of-line (BEOL) compatible conditions having a limited thermal budget (&lt;400^oC maximum temperature) must be addressed.<br/>Metal oxide semiconductors incorporating In₂O₃, have received significant attention as they possess high mobility, high I_ON/I_OFF ratio, ultra-scaled gate length, and sufficient drive currents. Doping In₂O₃ with several mole % tungsten oxide to form “IWO” has been reported to improve the threshold voltage stability of these semiconductor channel materials. However, currently reported film deposition for IWO is limited to sputtering, which cannot grow conformal films on topographically complex surfaces. This limitation becomes critical when utilizing these films as channel materials in gate-all-around or FinFET structures, which inherently possess 3D geometries. To address the increasing demand for high on-current, the shift towards 3D structures is necessary.<br/>Consequently, we use the 3D-compatible atomic layer deposition (ALD) method to deposit IWO by adjusting the cycle ratios for In and W precursors and the oxidant co-reactant, enabling precise control over the doped film composition. We demonstrate deposition of IWO (1~4 mol% WO_X) films using ALD and fabricate both bottom- and top-gated thin film transistors with a 3-nm thick IWO channel in a BEOL-compatible process with a maximum temperature of less than 250^oC. The 1 mol% WO_X-doped IWO transistor exhibits exceptional performance characteristics, including a subthreshold slope of 65 mV/decade, a high I_D,sat of 70 (W=12, L=1.5 ) at V_DS=1.0 V and V_GS=2.0 V, and remarkable stability under bias stress. The transistor showed negligible hysteresis and maintained excellent threshold voltage (V_th) stability under negative and positive bias stress conditions (ΔV_th= -0.06, and +0.1 V, respectively, at an electric field of 4.2 MV/cm for 1000 seconds). This V_th stability under bias stress highlights the reliability of ALD-grown IWO for ferroelectric field-effect transistors and its potential to enable high-performance monolithic 3D integrated devices.