Highly Reliable Polysilsesquioxane Passivation Layer fora-InGaZnO Thin-Film Transistors

Abstract

We report the fabrication of highly reliable amorphous InGaZnO (a-IGZO) passivated by a polysilsesquioxane-based passivation layer using a simple solution process. Results show that a copolymer of methylsilsesquioxane and phenylsilsesquioxane is an effective passivation layer. a-IGZO thin film transistors (TFT) passivated by this copolymer showed a small threshold voltage (V th ) shift of 0.1 V during positive bias stress, very small V th shift of less than 0.1 V during negative bias stress and a minimal V th shift (∼ −2.4 V) during negative bias illumination stress. These results demonstrate the potential of easy to fabricate polysilsesquioxanebased passivation layers as effective passivation materials.Amorphous In-Ga-Zn-O (a-IGZO) has become a popular active channel material for thin-film transistors (TFT) because of properties such as low fabrication temperature, high mobility, low threshold voltage (V th ), small subthreshold swing (S) and large area uniformity. 1 Because of these impressive properties, a-IGZO is expected to replace conventional amorphous Si as channel material. However, instability during bias stress is a problem especially for unpassivated bottom gate type TFT. Ambient environmental effects, moisture, desorption of oxygen and water, and post-fabrication processes can harm the exposed back channel leading to the degradation of reliability. 2 Lee et. al. previously discussed how moisture can affect the negative bias thermal instability of a-IGZO TFT due to either the creation of water related defects that act as electron traps or the action of water as a shallow donor. 3 Park et. al. also examined the effect of adsorbed water on the back channel of an unpassivated TFT and claimed that the role of water molecules as an electron donor or in forming acceptor-like traps is thickness-dependent. 4 In addition, they showed that adsorbed oxygen on the back channel acts as an electron acceptor which leads to a positive V th shift ( V th ). 4 Several groups have already employed passivation layers such as SiO x , 5 TiO x , 6 SiN x , 7 and Al 2 O 3 , 8 among others, to address the problem. These passivation layers have been shown to be effective in reducing instability during standard positive and negative bias stress by inhibiting the degradation effects discussed earlier. Nevertheless, more work is needed to address the more pressing issue of instability after application of negative bias illumination stress (NBIS). Moreover, these passivation layers are usually coated using a more complicated vacuum process. In this research, we report the effect of using a polysilsesquioxane-based passivation layer fabricated using a simple solution process on a-IGZO TFT. The passivated TFT showed good stability during positive bias stress (PBS) and a very good stability during application of negative bias stress (NBS) and negative bias illumination stress (NBIS). ExperimentalTFT fabrication and passivation.-N-doped Si substrates (resistivity < 0.002 · cm) with a thermally oxidized layer of 100 nm SiO 2 was chosen a...

Highly reliable passivation layer for a-InGaZnO thin-film transistors fabricated using polysilsesquioxane

“…We also observed that hump effect is less pronounced for both NBIS instability is largely caused by oxygen vacancies (V O ) [18]. We have recently reported that a high C and H concentration in the a-IGZO layer can contribute to the higher reliability especially during NBIS [15]. …”

“…More details about the polysilsesquioxane passivation layers used in this work is reported in Ref. [15].…”

“…Although organic passivation layers can be fabricated more easily compared to their inorganic counterparts, most that have been reported have large V th shifts ( V th ) during bias stress. We have recently reported an organic passivation layer based on polysilsesquioxane that is not only easy to fabricate but also highly reliable [15]. In this work, we will show the effect of reactive ion etching (RIE) on a-IGZO TFTs passivated with polysilsesquioxane and that these TFTs have high stability after post annealing.…”

Nano-crystallization in ZnO-doped In2O3 thin films via excimer laser annealing for thin-film transistors

“…The lower energy peak around 530 eV indicates the oxygen ion peak originating from the metal-oxygen bond (M-O), the peak around 531.5 eV indicates the oxygen deficient region (O def ), and the peak around 532.3 eV indicates a weak bond such as O-H (O weak ). [19][20][21][22] Figure 7 shows the plots of area ratio calculated from the curve-fitted O1s peaks, and shows the slight but definite decrease in O def and increase in M-O with increasing annealing temperature.…”

Analog-to-digital converter with oxide thin-film transistors

“…The developed ADC consists of an inverter-based comparator and an encoder with n-channel a-IGZO TFTs. The TFTs have a bottom gate structure with a methyl-siloxane-based organic spin-on-glass (SOG) passivation layer, which reduces the plasma damage on the active layer [19]. After examining comparators and logic gates with a-IGZO TFTs, a three-bit ADC was developed.…”