Process Investigations on Sputter Deposited Indium Tungsten Oxide TFT

Indium-Tungsten Oxide (IWO) has been investigated as a potential semiconductor material for next-generation display devices. Several publications have reported on IWO thin-film transistors (TFTs) with a channel mobility μch ~ 20cm2/Vs, approximately 2X higher than typically reported for Indium-Gallium-Zinc Oxide (IGZO). However, other attributes of the device characteristics have not performed as well as IGZO such as the subthreshold behavior. This work presents a study on the electronic properties of sputtered IWO, with the sputter ambient (i.e. percent O2 ) and O2 annealing conditions as the primary factors investigated. In the early stages of the study, a problem with the IWO target surface condition was found to be responsible for non-reproducible TFT properties. This initiated an extensive investigation on the details of target reconditioning, presputter conditions and reactive sputter ambient with oxygen. IWO sputter recipes and procedures were implemented which ensured consistent plasma characteristics and target surface condition. Reactive sputter conditions with low O2 content resulted in electrical behavior that was very different from argon-only sputtering (zero percent oxygen). IWO TFTs processed with 1.2% O2 sputter ambient, followed by O2 annealing at 300°C for 8-10 hours demonstrated promising current-voltage characteristics, with a low-field channel mobility μch ~ 18 cm2/Vs @ VDS = 0.1V. While higher oxygen content sputter conditions (10% O2) demonstrated a higher post-anneal resistivity, the device operation was inferior to the low oxygen treatment TFT. This was apparent in both on-state and off-state conditions, with the low oxygen treatment exhibiting higher current drive and steeper subthreshold. Increasing the O2 anneal temperature to 350°C on TFTs with the low oxygen treatment showed encouraging gate-controlled channel modulation for a 2 hour anneal. Raising this temperature to 400°C resulted in reduced gate control and current modulation, suggesting the onset of an additional defect mechanism. Degradation in device operation demonstrated by the high oxygen treatment as-sputtered material, or by aggressive O2 annealing at T > 350°C, may have a similar origin.