Enhancement-Mode Atomic Layer Deposited W-Doped In2O3 Transistor at 55 nm Channel Length by Oxide Capping Layer with Improved Stability

Amorphous oxide semiconductor field-effect transistors (AOSFETs) exemplify the tradeoff between mobility, stability, and threshold voltage $(\mathrm{V}_{\text{TH}})$. In this work, a new 5-axes AOSFET evaluation framework for backend-of-line (BEOL) integration is proposed, including (i) $\mathrm{I}_{\mathrm{D}}$ extracted at a fixed over-drive beyond $\mathrm{V}_{\text{TH}}$ at 1 pA/um for performance, (ii) $\mathrm{I}_{\text{OFF}}$, (iii) $\mathrm{V}_{\text{TH}}$, (iv) subthreshold slope (SS) at 1V $\mathrm{V}_{\text{DS}}$ for off-state behaviors, and (v) $\mathrm{V}_{\text{TH}}$ shift under positive bias stress for stability. To break the tradeoff between mobility and $\mathrm{V}_{\text{TH}}$, an oxide capping layer and post-capping anneal are used on back-gated W-doped $\text{In}_{2}\mathrm{O}_{3}$ (IWO) FETs. The oxide capping and anneal demonstrate stoichiometry-independent positive $\mathrm{V}_{\text{TH}}$ shift on 1% and 2% IWO channel FETs by 0.85V and 0.4V, respectively, while mobility increases from 18.5 to 26 $\text{cm}^{2}\mathrm{V}^{-1}\mathrm{s}^{-1}$ for 1% IWO. The contact resistance is also lowered from 2185 $\Omega-\mu \mathrm{m}$ to 967 $\Omega-\mu \mathrm{m}$, enabling $\mathrm{I}_{\text{ON}}$ increase by $1.42\times$. With the oxide capping and anneal, the stability under positive bias stress improves by 300 mV to -67 $\text{mV V}_{\text{TH}}$ shift. An enhancement-mode 1% IWO FET is shown at 55 nm $\mathrm{L}_{\text{CH}}$ with positive $\mathrm{V}_{\text{TH}}=0.53\mathrm{V}$, low $\mathrm{I}_{\text{OFF}}=160\ \text{pA}/\mu \mathrm{m}, \mathrm{I}_{\mathrm{D}}=192\ \mu \mathrm{A}/\mu \mathrm{m}$ at 1E13 cm⁻² charge density, and $\mathrm{I}_{\mathrm{D}}=50\mu \mathrm{A}/\mu \mathrm{m}$ extracted at a fixed over-drive voltage beyond 1 pA/um at 1V $\mathrm{V}_{\text{DS}}$.