用于透明AMOLED 的新型氧化物薄膜晶体管

1. Introduction 

Flexible displays based on organic light emitting  diodes (OLED) have attracted interests because they  are considered to create new applications of  electronics. The reliable transparent TFT array  showing good electrical performance is indispensable  and it should be manufactured at low temperature for  the flexible displays which are commercially available.  In recent time, the flat panel display televisions  demand high resolution, large size, fast response, and  low power consumption. As the display size and  resolution increases, the TFT back-plane with good  uniformity, low cost and high stability is required.[1] Conventional a-Si (amorphous silicon) TFT can be  fabricated with high uniformity and low cost, however,  its mobility and bias stability are poor. LTPS (low  temperature poly silicon) TFT has disadvantages for  large size production due to relatively poor uniformity  and high cost.[2] For these reasons, oxide TFTs are  considered as prominent candidate for the driving  device of AMOLED.[3] The transparent oxide TFTs  using ZnO[4], In-Zn-O[5-6], Zn-Sn-O[7-8] and IGZO[9-10] as an active channel material have been widely  studied. Most oxide TFT is required high temperature  (>200o C) processing to represent a good electrical  performance. Several studies concerning about the  oxide TFT processed at low temperature (<150o C) were reported, however the electrical performances  were relatively poor.[11] In order to use plastic substrates for flexible displays, the processing  temperature lower than 180o C for all manufacturing  process is desirable.

2. Experimental  

We have fabricated the inverse co-planar type  bottom gate TFTs with the AZTO active layer  sputtered at room temperature. Sputtering method has  advantages on low cost and large area uniformity  among various deposition methods. The active  material was composed of Al2O3-ZnO-SnO2. The  schematic diagram of the AZTO TFT structure is  shown in figure 1. A 100x100 mm2  alkaline-free glass was used as a substrate after the ultrasonic cleaning  with acetone, iso-propyl alcohol and DI water in  sequence. Gate and source/drain electrodes were  constituted with 150 nm thick ITO. A gate  insulator of Al2O3 was formed by atomic layer  deposition (ALD) method at 150ଇ and its thickness  was 185 nm. An AZTO layer was formed by cosputtering of an AlOx-ZnO (AlOx 2 wt%) target and a  SnO2 target (ANP Co.) with an off-axis type RF  magnetron sputter at room temperature. The sputtering  was performed in the atmosphere of Ar and O2 mixed  gas with the chamber pressure of 0.2 Pa. All  patterning processes were performed with photolithographic method and wet etching process. The  post-annealing was performed in vacuum using  electric ovens. The electrical characteristics of the  TFTs were measured with the semiconductor  parameter analyzer (Agilent B1500A). X-ray  diffraction (XRD) spectra of the AZTO films were  recorded with a Rigaku RU-200BH diffractometer  XVLQJ &X.Į UDGLDWLRQ The chemical composition of  AZTO thin film was analyzed by Auger electron  spectroscopy method.

3. Results and discussion  

The AZTO thin film deposited by RF magnetron  sputtering at room temperature was amorphous.  Figure 2 shows XRD spectra of AZTO thin films  having composition of about 4 mol% AlOx, 66 mol%  ZnO, and 30 mol% SnO2 before and after annealing at  300o C for 1 hour. There was no diffraction peak of  crystalline phase in the XRD spectra even after the  300o C annealing. Thus, the AZTO active layers on the  TFTs were considered as very stable amorphous oxide  material. Amorphous oxide active layer has an  advantage on large area uniformity and long-term  reliability because it has no grain boundary.

Fig1

4. Summary 

We have manufactured the AMOLED panel with  the new oxide TFT back-plane composed of the  AZTO active layer which was deposited at room  temperature and annealed at low temperature. The  room temperature deposition and the low temperature  process are significant advantages for the fabrication of flexible electronics. The AZTO layer was deposited  by sputtering which is convenient for large size  commercial production so that the AZTO TFT is  considered to be a prominent candidate for the driving  device of large size flexible displays.