Highly reliable passivation layer for a -InGaZnO thin-film transistors fabricated using polysilsesquioxane
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Highly reliable passivation layer for a-InGaZnO thin-film transistors fabricated using polysilsesquioxane Juan Paolo Bermundo1, Yasuaki Ishikawa1, Haruka Yamazaki1, Toshiaki Nonaka2, and Yukiharu Uraoka1 1 Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan 2 AZ Electronic Materials Manufacturing Japan K.K. 3330 Chihama, Kakegawa-shi, Shizuoka, 437-1412, Japan ABSTRACT Polysilsesquioxane passivation layers were used to passivate bottom gate a-InGaZnO (a-IGZO) thin film transistors (TFT). The a-IGZO TFTs passivated with polysilsesquioxane showed highly stable behavior during positive bias stress, negative bias stress, and negative bias illumination stress. A voltage threshold shift of up to 0.1 V, less than -0.1 V and -2.3 V for positive bias stress, negative bias stress, and negative bias illumination stress, respectively. We also report the effect of reactive ion etching (RIE) on the electrical characteristics of a-InGaZnO (a-IGZO) thin-film transistors (TFT) passivated with the polysilsesquioxane-based passivation layers. We show how post-annealing treatment using two different atmosphere conditions: under O2 ambient and combination of N2 and O2 ambient (20% O2), can be performed to recover the initial characteristics. Furthermore, we present a highly stable novel polysilsesquioxane photosensitive passivation material that can be used to completely circumvent the reactive ion etching effects. INTRODUCTION Amorphous InGaZnO (a-IGZO) has been extensively studied as replacement for a-Si channel materials in thin film transistors [1-3]. This is not only due to its higher mobility, lower threshold voltage (Vth) and much lower off current compared to a-Si but also because of other remarkable properties such as low temperature fabrication, good uniformity and transparency. This makes a-IGZO attractive in current display applications such as organic light emitting diode displays [1,4] and in future display devices where flexibility and transparency are necessary [5]. However, despite having impressive properties, a-IGZO TFTs especially those having a bottom gate structure suffer from instability because the backchannel is exposed to ambient effects. The influence of these ambient effects such as moisture, adsorbed oxygen and post fabrication damage have been extensively discussed [6-9]. The proposed solution to this problem has been to either adopt a different sample structure, for example: top gate structure [2] and etch stop structure [10] or to coat the TFT with a passivation layer. Both inorganic passivation layers (Al2O3 [11], SiOx [12], SiNx [13]) and organic passivation layers such as CYTOP [14] and photoacryl [9] have been proposed. Nevertheless, there are some disadvantages that arise from these proposed solutions. For instance, using a different structure such as top gate structure
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requires additional mask and patterning process steps. Similarly, inorganic passivation layers require much more complicated vacuum processes. Althoug
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