1. Field of the Invention
An aspect of the present invention relates to a sputtering apparatus for depositing an extremely low concentration of a metal catalyst on an amorphous silicon layer in order to crystallize the amorphous silicon, and more particularly, to a sputtering apparatus capable of minimizing non-uniformity of a metal catalyst caused by a pre-sputtering process without reducing process efficiency, and thus improving uniformity of the metal catalyst deposited on an amorphous silicon layer at an extremely low concentration.
2. Description of the Related Art
Flat panel display devices have replaced cathode ray tube display devices due to characteristics such as light weight, thin thickness, and so on, and typical examples thereof include liquid crystal displays (LCDs) and organic light emitting diode (OLED) display devices. In comparison with the LCDs, the OLED display devices are excellent in brightness and viewing angle characteristics, and require no backlight, so that OLED display devices can be realized as ultra thin displays.
The OLED display devices are classified into two types, a passive matrix type and an active matrix type, according to the driving method. The active matrix type OLED display devices include a circuit using a thin film transistor (TFT).
The TFT generally includes a semiconductor layer, which has a source region, a drain region and a channel region, and gate, source and drain electrodes. The semiconductor layer may be formed of polycrystalline silicon (poly-Si) or amorphous silicon (a-Si). The poly-Si has a higher electron mobility than the a-Si. Thus, the poly-Si is mainly used for the TFT of OLED display devices.
Among the methods of crystallizing the a-Si into the poly-Si, one uses a metal. The crystallizing method using the metal involves depositing a metal catalyst on a substrate using a process such as a sputtering process of depositing a metal layer on a substrate by applying plasma to a metal target formed of a crystallization-inducing metal such as nickel, or an atomic layer deposition (ALD) process of forming an atomic layer of a crystallization-inducing metal catalyst such as nickel on a substrate using a chemical method based on a reaction gas containing the metal catalyst, and crystallizing the a-Si using the metal catalyst as a seed, thereby providing an advantage in that the a-Si can be crystallized at a relatively low temperature in a short time.
Typical sputtering apparatuses are designed to concentrate the plasma on the metal target and substrate using a magnetic assembly located at the rear of the metal target to uniformly deposit a thick layer in a short time. However, the crystallizing method using the metal degrades the characteristics of the TFT when the metal catalyst remains in the poly-Si formed by crystallizing the a-Si, and particularly, the TFT may be driven unstably when the metal catalyst remaining in the poly-Si is magnetized. Therefore, sputtering apparatuses based on the crystallizing method using the metal perform a deposition process while moving the metal target made of the metal catalyst without using the magnetic assembly to be able to prevent magnetization of the metal catalyst discharged from the crystallization-inducing metal such as nickel as well as to deposit the metal catalyst at an extremely low concentration.
In the sputtering apparatus that performs the deposition process while moving the metal target without using the magnetic assembly, when a pre-sputtering process of removing foreign materials such as NiO2 attached to a surface of the metal target prior to the deposition process is performed to be able to more uniformly and stably deposit the extremely-low-concentration metal catalyst, the metal catalyst discharged from the metal target during the pre-sputtering process is deposited on an edge of the substrate, so that the uniformity of the metal catalyst deposited on the substrate can be reduced.
Further, to prevent the metal catalyst discharged from the metal target during the pre-sputtering process from being deposited on the substrate, a region where the plasma is produced is isolated during the pre-sputtering process. In this case, a separate shield is required to isolate the plasma production area. Further, it takes a predetermined time to form or remove the separate shield depending on progress of the pre-sputtering process, so that the deposition process is delayed, and thus overall process efficiency is reduced.