Hydrogenation in laser annealed polysilicon thin-film transistors (TFTs)
Introduction
Polycrystalline silicon thin-film transistors (polysilicon TFTs) are widely investigated, mainly due to their application in active-matrix liquid crystal displays (AMLCDs). The performance of polysilicon TFTs depends strongly on defects in the polysilicon and at the polysilicon/gate oxide interface [1]. Recently, laser-annealing techniques were applied to improve the polysilicon TFT performance through grain-size enlargement and reduction of the grain boundary and in-grain defect density [2]. A traditional technique to reduce defects in TFTs is hydrogenation. Various hydrogenation techniques are currently applied, such as plasma hydrogenation [3], hydrogen implantation [4] and SiNx:H encapsulation [5]. An important issue of the hydrogenation process is its duration, resulting in improvement of the turn-on voltage, carrier mobility and sub-threshold swing, and in the dispersion of the static device characteristics.
In this work, we used plasma hydrogenation, with the hydrogen plasma formed by a low % hydrogen diluted with a rare gas, namely helium or argon. The aim of this work was to investigate the effect of various hydrogen concentrations, diluted with helium or argon, on the device performance, the dispersion of the device parameters and the device reliability.
Section snippets
Experimental
The studied polysilicon TFTs were fabricated on glass substrates by a low-temperature process described elsewhere [2]. Amorphous silicon (α-Si) films 51.8 nm thick were deposited by low-pressure chemical vapor deposition (LPCVD) at 425°C and 1.1 torr, using Si2H6 as the reactant gas. Then, the α-Si films were crystallized by XeCl excimer laser (λ=308 nm, 14 shots) with energy density 370 mJ cm−2. After forming a 121-nm thick SiO2 gate-insulator by electron cyclotron resonance (ECR)-PECVD at
Static device parameters
Fig. 1 shows the device parameters as a function of the H2 concentration diluted in He or Ar. It is apparent that VON is improved when a plasma of 2–4% H2/Ar is used for hydrogenation, while a high concentration of H2 is required for the improvement of VON when a plasma of H2/He is used. After hydrogenation in a plasma of 4% H2/He, the devices exhibit a VON value approximately 1 V lower than the unhydrogenated ones, and even lower than those hydrogenated with 100% H2. Concerning the effective
Conclusions
The effects of various hydrogenation processes were investigated in polysilicon TFTs. The device parameters are improved after hydrogenation in a hydrogen plasma formed by H2 diluted with He or Ar. In addition, the dispersion of device characteristics is improved after hydrogenation. However, hot-carrier stress experiments demonstrate that hydrogenation severely affects the device reliability. Furthermore, it was found that a hydrogen/helium mixture gives more reliable transistors than
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