1. Field of the Invention
The present invention relates to a method for fabricating an organic thin film transistor, and more particularly, to a method for fabricating an organic thin film transistor and a method for fabricating a liquid crystal display device using the same that employ an rear exposing process to form an active layer.
2. Discussion of the Related Art
Following the development of polyacetylene, which is a conjugated organic polymer exhibiting semiconductor characteristics, an organic semiconductor has been actively studied because its advantageous characteristics of easiness in formation of a film type, flexibility, conductivity and a low production cost. An organic semiconductor may be employed in an electronic device or an optical device.
Among devices using the semiconductive polymer, an organic thin film transistor (OTFT) employing an organic material has been the focus of many ongoing researches. In general, an OTFT has a similar structure as that of an Si-TFT, and an OTFT employs an organic material at a semiconductor region, instead of Si.
An OTFT has many advantages in that a thin film can be formed by an atmospheric pressure printing process instead of plasma enhanced chemical vapor deposition (PECVD) requiring sub-atmospheric pressure for forming the existing Si thin film, a roll-to-roll process using a plastic substrate can be performed, and a low-cost transistor can be implemented.
FIGS. 1A to 1E are cross-sectional views showing a method for fabricating an organic thin film transistor (OTFT) in accordance to the related art. In FIG. 1A, a method for fabricating an OTFT according to the related art includes preparing a transparent substrate 10. In particular, a first metal material is deposited and patterned to form a gate electrode 11 using a photolithography process. The photolithography process includes a photoresist film coating process for coating a photoresist film on an etching object layer on which a pattern is to be formed, an exposing process for aligning a mask on the photoresist film and irradiating light through the mask, a developing process for forming a photoresist pattern on the etching object layer by removing the irradiated portions of the photoresist film using a developer, an etching process for forming a desired pattern by etching the etching object layer by using the photoresist pattern as a mask, and a striping process for removing the photoresist pattern remaining on the pattern. For example, the first metal material is the etching object layer, and the gate electrode 11 is patterned as the first metal material is etched.
Subsequently, SiNx or SiOx is deposited on the entire surface of the first substrate 10 including the gate electrode 11 using a plasma enhanced CVD method to form a gate insulating layer 13.
In FIG. 1B, a low-molecule organic material, such as pentacene, is deposited at an upper surface of the gate insulating layer 13 to form an active pattern 15 on a portion of the gate insulating layer 13 corresponding to the gate electrode 11. Because a photoresist film changes electric characteristics of the pentacene, a conventional photolithography process cannot be used. Thus, the active pattern 15 is formed using a shadow mask. The shadow mask has a certain region opened for allowing formation of a pattern and has a different concept from a mask used for a conventional exposing process. For example, the mask used for the conventional exposing process is divided into regions for blocking light or transmitting light, whereas the shadow mask is divided into the opened region and a closed region, so that a material for forming a pattern is deposited only through the opened region to form the same pattern as the opened region. However, a pattern formed using the shadow mask has a quite inferior precision as compared to a pattern formed by using the general mask.
As shown in FIG. 1C, a second metal material is deposited on the active layer 15 and is patterned to form source and drain electrodes 16 and 17 on the active layer 15. In particular, the second metal material is patterned using the photolithography process to form the source and drain electrodes 16 and 17 like the gate electrode 11.
In FIG. 1D, an inorganic film or an organic film is deposited at the entire surface of the substrate 10 including the source and drain electrodes 16 and 17 to form a passivation film 18. The passivation film 18 is patterned to form a contact hole 19 exposing a portion of the drain electrode 17 using the photolithography process.
Further, as shown in FIG. 1E, a transparent conductive material, such as ITO, is deposited on the entire surface of the substrate 10 including the contact hole 19, and is patterned to form a pixel electrode 20 electrically connected to the drain electrode 17 through the contact hole 19. The transparent conductive material is patterned using the photolithography process to form the contact hole 19.
However, because the method of fabricating an OTFT according to the related art requires a shadow mask to form the active pattern, the accuracy of the active pattern reduces and the number of masks uses increases. Thus, a fabrication efficiency decreases, and electrical characteristics of the OTFT are degraded.