Study on the IGZO Ceramics Sintered at Different Temperatures

In this study, the IGZO target was synthesized through a pressureless oxygen atmosphere sintering technique, and the effects of sintering temperature on IGZO ceramic target were studied. The In2O3, Ga2O3 and ZnO powders in the mole ratio of 1:1:2 were selected as raw materials. The powders were mixed by ball milling, and then the granulation and pressing process were used in order to obtain the green compacts. Then the green compacts were sintered at different temperature under oxygen atmosphere. The microstructure characterizations and compositions of the IGZO ceramic targets were analyzed. The results indicated that the lower sintering temperature was beneficial for IGZO ceramics to form the regular polygonal grains. With the temperature increased, the densification of IGZO ceramics was highly activated, and the low porosity was obtained. The XRD results demonstrated that the single phase of InGaZnO4 had been generated at 1100°C and no phase transformation occurred between 1300°C and 1500 °C. However, the SEM results showed that the grain growth of IGZO target was very obvious at 1500°C. The IGZO ceramic had a highest relative density of 99.4% and optimal resistivity of 18 mΩ·cm at the sintering temperature of 1400°C.


Introduction
The continuous advances in display technologies have prompted considerable interest in flexible displays, electronics and transparent electrodes [1][2]. Moreover, the amorphous indium-galliumzinc(α-IGZO) thin-films have received much attention in the field of thin film transistors (TFT), which is the indispensable device for making flat panel display [3]. Compared with the α-Si and LTPS (Low Temperature Poly-silicon) TFT (Thin Film Transistor), IGZO-TFT has two obvious advantages [4]. On the one hand, the leakage current of the IGZO transistor is much lower than LTPS or α-Si technology [5].On the other hand, the IGZO can pass 20-50 times more electrical current than equivalent amorphous silicon TFT [6]. Therefore, it is necessary to study high performance IGZO ceramics for preparing IGZO thin films [7][8].
With the development of flat panel display industry, the development of IGZO target shows the following trends [9][10][11]: (1) Low resistivity: transparent conductive films with smaller resistivity are needed due to the high requirements on the driver device. Therefore, the resistivity of the IGZO targets should be as lower as possible. (2) High density (relative density > 99%): the low target density will The 11th International Conference on High-Performance Ceramics IOP Conf. Series: Materials Science and Engineering 678 (2019) 012117 IOP Publishing doi: 10.1088/1757-899X/678/1/012117 2 decrease the effective sputtering area, and then the sputtering speed will decrease. At the same time, the high density target can prolong the service life of target and reduce application cost.(3) Improved utilization rate: Improving target utilization rate has always been the direction from equipment manufacturers, users and target manufacturers. The design of magnetron sputtering magnets moving periodically from front and back, the improvement of sputtering parameters, and the development of tubular target greatly increased the utilization rate of the targets [12][13].
In order to prepare high performance IGZO films, it is necessary to study high performance IGZO targets as sputtering materials [14][15]. At first, in order to optimize the performances of sputtered films, the correlations between target performance and film properties must be thoroughly clarified [16]. For the oxide targets, these factors are critical to the performance of the target, such as, the activity of powders, sintered density, grain size and electrical properties of targets can obviously affect the sputtering process and the film properties. Meiet al. [17] investigated the effects of particle size and dispersion methods of In 2 O 3 -SnO 2 mixed powders on the properties of ITO ceramics, the study demonstrated that the dispersion of mechanical ball-mill methods for nanosized In 2 O 3 and SnO 2 powders is beneficial to the densification and structural homogeneity, and the smaller the relative grain size, the more uniform the distribution of grain size. Furthermore, Han et al. [18]invesrigated the densification and grain growth of ZnO doped with Al during isothermal sintering between 1100 and 1400 C. In this study, a mechanism of pore surface drag (pinning) on densification equivalent to the observed drag (pinning) of grain boundaries on grain growth was proposed. Ma et al. [19] fabricated ITO sputtering targets using In 2 O 3 -SnO 2 mixed powders and tin doped indium oxide powders, respectively. Meanwhile, the differencein microstructure and sintering mechanism, especially grain size distribution, grain morphology, major element distribution and sintering rate, were studied.Lee et al. [20]found that when the O 2 flow rate was zero, the results of a Hall measurement of a-IGZO films deposited from targets with higher Zn atomic ratio showed lower resistivity, higher carrier concentration, and lower mobility. Nevertheless, as the O 2 flow rate was more than 5 sccm, the result was opposite.
For the preparation of IGZO ceramics, the In 2 O 3 , Ga 2 O 3 , and ZnO powders are the raw materials. Then, the ball milling, drying, forming and sintering technology were used for obtaining the final product of IGZO target. However, little research has been done on composition transformation and densification process of IGZO ceramics during sintering. In the sintering process of target material, the changes of microstructure and phase transformation are complicated. Therefore, it is necessary to study the sintering process of IGZO ceramics at different temperature for improving the properties of IGZO ceramics. In this study, the influences of sintering temperature on the sintering process, microstructures, and electrical properties of IGZO targets are clarified.

Powder processing and forming
The In 2 O 3 , Ga 2 O 3 and ZnO powders were mixed in the mole ratio of 1:1:2. The purity of the powders was 99.99%, and the median size of In 2 O 3 , Ga 2 O 3 and ZnO were 40nm, 3μm and 0.2μm, respectively. The raw material powders with the above proportion were put in the zirconia ball mill. Water and organic dispersant were added to improve the ball milling effect of the slurry. And then the high energy planetary ball milling technology was used for ball milling process. The rotating speed of ball mill was 200r/min, and the ball milling time over 48 hours. In order to improve the cohesiveness of spray-dried powder, polyvinyl alcohol (PVA)of0.8 wt% was added into the slurry and milled for one additional hour. During ball milling, the solid content of slurry was about 60%.
Then the previous slurry was spray-dried to improve the liquidity of mixed powders. Thus, the pressing properties of mixed powders can be improved. The drying temperature of slurry was 200C and the feeding speed was about 5ml/min. Then, the spray dried powders were obtained and the size ranged from 10 to 30μm, which have good fluidity and filling property.