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

Metal oxide semiconductors incorporating In₂O₃ have received significant attention as they possess high electron mobility, can be synthesized at back-end-of-line (BEOL) compatible temperatures, and exhibit excellent transistor characteristics at nanoscale dimensions. 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 does not allow the growth of conformal films with precise thickness and stoichiometry control on topographically complex BEOL surfaces. This limitation becomes critical when utilizing these films as channel materials in gate-all-around structures. A shift toward deposition methods that support 3D structures is necessary to address the increasing demand for high on-current.<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. In addition to facilitating highly conformal film growth, ALD-grown IWO may exhibit different point defect populations and a broader process window for amorphous structure because of the lack of bombardment by energetic species that are present during sputtering. 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. A transistor with 2 mol% WO_X-doped IWO 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 a stable threshold voltage (V_th) under negative and positive bias stress conditions (ΔV_th= -0.06 V 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.<br/>Furthermore, we explored the evolution of long-range order within the IWO films through synchrotron X-ray diffraction. We found that doping In₂O₃ with several mole % of tungsten oxide has a strong anti-correlation with both the ordering and the presence of oxygen vacancies, which are themselves strongly correlated with V_th. This work offers an understanding of the impact of W doping and provides insight into the reliability of In₂O₃-based oxide transistors.