Non-Traditional Response to Intensive NBS and NBIS Treatments on IGZO TFTs
Authors: Powell Eli , Zhu Bin , Manley Robert G. , Hirschman Karl D. 
Published: December 2024

The traditional response to bias-stress treatments is consistent with the Fermi level position during the stress application. Positive-bias stress (PBS) supports trap state filling of electrons, presenting negative interface charge and causing a left-shift and/or distortion in the transfer characteristic. Negative-bias stress (NBS) supports trap state emptying of electrons, appearing as a net positive charge and causing a right-shift and/or distortion in the transfer characteristic. The extent of shifting, distortion, and hysteresis resulting from the bias-stress application depends on the magnitude of the bias-stress voltage and time duration, as well as the post-stress test conditions such as VGS sweep rate and temperature. Hysteresis occurs when the level of trapped charge is not in thermal equilibrium with the temperature and bias conditions, and provides insight into the magnitude of trap states involved in dynamic state changes.

This work provides an interpretation of defect and trap state behavior following the application of intensive NBS and negative-bias illumination stress (NBIS). Staggered bottom-gate TFTs were fabricated with a 50 nm IGZO film sandwiched between a 50 nm gate oxide and a 50 nm passivation oxide layer. The working metal was molybdenum for the gate electrode and source/drain contacts. A passivation anneal was done and an alumina capping layer was deposited via ALD to promote electrical stability; full process details are provided in a previous report. Devices were tested with a Keysight B1500 parameter analyzer to establish initial device characteristics, with +20 V PBS and -10 V NBS stress applications for 20 ks having a negligible response. Intensive negative-bias stress testing was then done for 10 ks at either high bias (-20 V NBS) or with the addition of illumination (-10 V NBIS @ 410 nm). The responses to intensive NBS and NBIS applications were both non-traditional and distinctive.

Pre- and post-NBS results are shown in Figure 1. ID-VGS transfer characteristics were measured over a VGS down-sweep from 10 V to -5 V at VDS values of 10 V (first) and 0.1 V (second), with a sweep rate of ~ 1 V/s. The measurement order, sweep direction and sweep rate were chosen to maintain consistent non-equilibrium conditions and suppress hysteresis. The intensive NBS shown in Figure 1a induces a significant distortion and spreading to the right, which can be explained by the creation of acceptor-like trap states during stress application. Figure 1b shows a comparison of hysteresis following NBS and subsequent PBS treatments, with the VGS up-sweep immediately followed by a down-sweep. Post-NBS hysteresis shows a separation of approximately 1 V, which translates to an equivalent interface state density difference of 4.3x1011 states/cm2. The separation in transfer characteristics reflects non-homogeneity in trap state formation. The application of +20V PBS following NBS was observed to partially reverse these effects, with hysteresis reduced significantly. The net shift around flatband is minimal, however pronounced distortion remains.

Pre- and post-NBIS results are shown in Figure 2. The intensive NBIS shown in Figure 2a induces a significant right-shift of around 6 V, with minimal distortion. While this can also be explained by the creation of acceptor-like traps, the states created in response to NBIS have a slow response (i.e. remain unchanged over VGS sweep) appearing more like fixed charge. Note that this large shift is observed in the hysteresis loop shown in Figure 2b, however once VGS is increased to +10 V the traps remain filled and hysteresis is dramatically reduced. The application of +20 V PBS following NBIS was observed to partially reverse these effects, with a left-shift of approximately -4 V, or to within 2 V of the original pre-NBIS characteristics. The hysteresis loop observed following PBS treatment and relaxation is consistent with the position of the post-NBIS hysteresis measurement (up-sweep) and the final transfer characteristic (down-sweep).

Remaining work in defect state characterization will utilize cryogenic measurements for temperature-dependent behavior. TCAD simulation will be used as a resource to model the defect state energy distribution resulting from the application of intensive NBS and NBIS treatments.

Figure 1