Low Temperature Polycrystalline Silicon Thin Film Transistors Fabricated by Nickel Organic Chemical Vapor Deposition

Abstract

Although the Active Matrix Liquid Crystal Display (AMLCD) has dominated the flat panel display market, Active Matrix Organic Light Emitting Diode (AMOLED) displays have recently begun to replace LCD displays for applications where a small screen is needed. It can be easily anticipated that AMOLED displays will soon be available in larger sizes as well, and so much research has been focused on Low Temperature Poly Silicon (LTPS). Metal-induced lateral crystallization (MILC) is a well-known LTPS process and MILC Thin Film Transistors (TFTs) have been known to show acceptable electrical properties for active matrix flat panel displays, especially for AMOLED displays. However, there still remain some problems to overcome, such as the relatively high leakage current, low electron mobility and thermal budget. The general consensus is that Ni silicides within the channel region of MILC poly-Si TFTs are responsible for these problems. In this work, 1-dimethylamino-2-methyl-2-butoxy nickel [C14H32N2O2Ni] was used as the Ni precursor called MABON (product name) to deposit an ultra-thin discontinuous layer of nickel on top of a-Si by Metal Organic Chemical Vapor Deposition (MOCVD), and crystallization was carried out at low temperature. By controlling the amount of nickel precursor in a carrier gas, it was possible to form nickel islands instead of a continuous thin layer, then it could be reduced the metal contamination in the channel area of Poly TFT prepared.

In order to reduce the amount of Ni silicide in the channel region, Ni was deposited on the amorphous silicon (a-Si) thin film by Metal Organic Chemical Vapor Deposition (MOCVD), using MABON as a precursor. The amount of Ni deposited on a-Si thin film was controlled by precursor flow and deposition time. When a-Si was crystallized by using MABON, the concentration of the Ni silicides in the MIC region could be reduced as compared with that of the MIC region crystallized by using the sputtered Ni film conventionally. Also, the electrical properties of MIC poly-Si TFTs fabricated using the novel process were investigated. The MIC poly-Si TFTs fabricated by using MABON exhibited a field-effect mobility of 44.0 cm2/Vs and on/off current ratio of 7.6 × 105, which were better than the values exhibited a field-effect mobility of 5.4 cm2/Vs and on/off current ratio of 1.8 × 105 by TFTs fabricated by using sputtered Ni film. And the TFTs fabricated by MABON were compared with the TFTs produced by conventional MILC. The TFTs fabricated by MABON showed a higher off current and a slightly lower on current than the conventional MILC TFTs. We conclude that a MABON reduce the Ni content in the channel region and the amount of Ni or Ni silicide is very significant factor, which causes a degradation of the MIC and MILC poly-Si TFTs, such as a high leakage current and a low field-effect mobility.

In the case of conventional MILC TFTs, it has been known that well-defined thin metal layers need for the crystallization of a-Si and locate away from the gate electrode of poly-Si TFTs. As regards Ni self-aligned MILC process, because Ni thin film was deposited on the whole source and drain region of poly-Si TFTs, so the junction regions which have to be depleted at off-state were crystallized by MIC. Therefore, the metal incorporation at the junction regions might cause leakage current at off state. To exclude the incorporation of metal contaminants by MIC, the boundaries of MIC and MILC were shifted from the junction regions to source/drain regions. So additional fabrication processes, such as photolithography, deposition, and etching steps are needed for Ni islands. In this study, to reduce the process steps which as mentioned above, we proposed a fabrication of self-aligned TFTs without the incorporation of metal contaminants by using MABON deposition. Using this method, we could crystallize without any additional process. Furthermore, it turned out that the poly-Si thin-films crystallized using this method contained Ni less than conventional sputtered Ni film MILC by SIMS analysis. The electrical properties of MABON self-aligned TFTs are found to be superior to those of Ni film self-aligned TFTs. The leakage current and field-effect mobility, which have been regarded as obstacles for industrialization of the MILC process, measure to be 3.4 × 10−11 A and 66 cm2/Vs, respectively.