1. Technical Field
The present disclosure relates to an organic light emitting diode (OLED) display device, and more particularly, to an OLED display device where a passivation layer has a smaller area than an upper electrode of a light emitting diode and a method of fabricating the OLED display device.
2. Discussion of the Related Art
Among various flat panel displays (FPDs), an organic light emitting diode (OLED) display device has superior properties such as high brightness and an ability to be driven by a low voltage. Since the OLED display device is an emissive type, the OLED display device has high contrast ratio and thin profile. The OLED display device has an advantage in displaying a moving image due to the low response time of several micro seconds (μsec). The OLED display device has no limitation on a viewing angle and is stable even in a low temperature. Since the OLED display device is driven by a low voltage of 5V to 15V in direct current (DC), it is easy to fabricate and design a driving circuit.
In addition, since a fabrication process of the OLED display device requires only a deposition apparatus and an encapsulating apparatus, the fabrication process of the OLED display device is simple.
The OLED display device includes a plurality of pixel regions, and a switching thin film transistor (TFT) and a driving TFT are formed in each of the plurality of pixel regions. Generally, the TFTs are formed by using a semiconductor material such as amorphous silicon.
Recently, to meet the requirements of large size and high resolution, the OLED display device including the TFTs having faster signal process, more stable operation and durability is required. However, the TFT using amorphous silicon has a relatively low mobility (e.g., less than 1 cm2/Vsec), which presents challenges in developing the OLED display device with large size and high resolution.
Accordingly, an oxide TFT including an active layer of an oxide semiconductor material, which has excellent electrical properties such as high mobility and low leakage current can be used to obviate some of these shortcomings.
Figure (FIG. 1) is a cross-sectional view showing an organic light emitting diode display device according to the related art, and FIG. 2 is a magnified view of a portion A of FIG. 1.
In FIGS. 1 and 2, an organic light emitting diode (OLED) display device 10 includes first and second substrates 20 and 50, an array layer 48, a second electrode 40 and a passivation layer 42 sequentially on the first substrate 20 and a seal pattern 44 at an edge portion between the first and second substrates 20 and 50.
The first and second substrates 20 and 50 facing and spaced apart from each other include a plurality of pixel regions (not shown). The array layer 48 includes a gate line (not shown), a data line (not shown), a power line (not shown), a switching thin film transistor (TFT), a driving TFT T, a first electrode 34 and an emitting layer 38. Also, the gate line, the data line and the power line cross each other to form the plurality of pixel regions. In addition, the switching TFT is coupled to the gate line and the data line. Moreover, the driving TFT T is coupled to the switching TFT, the power line, and the first electrode 34 is connected to the driving TFT T. Furthermore, the first electrode 34, the emitting layer 38 and the second electrode 40 constitute a light emitting diode (LED) D.
The array layer 48 is disposed in a display region at a central portion of the first and second substrates 20 and 50. A gate electrode 22 is formed in each pixel region on an inner surface of the first substrate 20, and a gate insulating layer 24 is formed on the gate electrode 22. Also, an oxide semiconductor layer 26 is formed on the gate insulating layer 24 corresponding to the gate electrode 22, and an etch stopper 27 is formed on the oxide semiconductor layer 26. In addition, source and drain electrodes 28 and 30 are formed on both end portions, respectively, of the etch stopper 27 and the oxide semiconductor layer 26. The gate electrode 22, the oxide semiconductor layer 26, the source electrode 28 and the drain electrode 30 constitute the driving TFT T.
A protecting layer 32 is formed on the driving TFT T. The protecting layer 32 includes a drain contact hole 33 exposing the drain electrode 30 of the driving TFT T.
The first electrode 34 coupled to the driving TFT T is formed on the protecting layer 32 in each of the plurality of pixel regions, and a bank layer 36 is formed on a boundary portion of the first electrode 34. The bank layer 36 includes an opening exposing a central portion of the first electrode 34. The emitting layer 38 contacting the first electrode 34 through the opening is formed on the bank layer 36, and the second electrode 40 is formed on the emitting layer 38. The first electrode 34, the emitting layer 38 and the second electrode 40 constitute the LED D. In addition, the passivation layer 42 is formed on the second electrode 40.
The seal pattern 44 attaches the first and second substrates 20 and 50, and a plurality of pads 46 coupled to a driving circuit (not shown) are formed on the first substrate 20 outside the seal pattern 44. The first substrate 20 is referred to as a lower substrate, a TFT substrate or a backplane, and the second substrate 50 is referred to as an encapsulation substrate.
In the OLED display device 10, each of the second electrode 40 where a common voltage is applied and the passivation layer 42 protecting the LED D from an exterior moisture or an exterior contaminant are formed as a single body covering the plurality of pixel regions. As a result, each of the second electrode 40 and the passivation layer 42 is formed on an entire surface of the first substrate 20.
The passivation layer 42 is formed by a plasma chemical vapor deposition (PCVD) apparatus or a physical vapor deposition (PVD) apparatus such as a sputter. In one exemplary embodiment, silicon nitride (SiNx) film, silicon oxynitride (SiON) film or silicon oxide (SiOx) film formed by the PCVD apparatus or alumina (AlOx) film formed by the sputter may be used as the passivation layer 42.
However, when the passivation layer 42 is formed of silicon compound by the PCVD apparatus, a deposition process should be performed under a relatively low temperature (e.g., less than about 100° C.) to prevent deterioration of the emitting layer 38. Due to the relatively low temperature, source gases do not completely react and hydrogen (H) from the source gases such as silane (SiH4) gas or ammonia (NH3) gas remains in the passivation layer 42.
The hydrogen is diffused into the oxide semiconductor layer 26 of the driving TFT T through the protecting layer 32 to generate a reduction process of the oxide semiconductor material of the oxide semiconductor layer 26. The reduction of the oxide semiconductor material due to the hydrogen results in a threshold voltage shift of the driving TFT T, and the threshold voltage shift causes deterioration such as a stain or brightness deviation in an image. As a result, a displaying quality of the OLED display device 10 is degraded.