Research On Poisoning And Doping Effects By Nb/Ti Of ITO Films

Indium tin oxide(ITO)film is the main coating material for Thin Film Transistor Liquid Crystal Display(TFT-LCD)due to its high conductivity and light transmittance.It is also applied to solar cells and flexible substrate devices gradually.At present,almost all the ITO targets used for TFT-LCD depend on import.In this work,the causes and mechanism of "poisoning" occurred during sputtering on the surface of ITO target were studied.It is proposed that the methods for reducing the "poisoning" of the targets and improving the photoelectric properties of the films through the study on the "poisoning" of Nb/Ti doped ITO(ITO:Nb/Ti)targets and the influence of sputtering parameters on the photoelectric properties of ITO:Nb/Ti films.Firstly,the reason of cracking of ITO target was studied.Results showed that the cracking was caused by thermal stress during RF magnetron sputtering at the area in which impurity condensing or composition aggregating when the target is not pure enough.Impurity condensing or composition aggregating would result in resistivity increasing,thermal conductivity performance degradation,and then the cohesive strength between the grains of target reducing.At the same time,it is stated that the same mechanism results in the deposition of clusters of particles deposited on the surface of the target material and forming the nodules.Secondly,the further causes of the "poisoning" of ITO target(including ITO:Nb/Ti target)and the influence of poisoning on the main properties of ITO films were discussed.The results show that target density and purity of ITO target are the main factors influencing the degree of poisoning.For the target with the purity higher than 99.9%and the relative density greater than 99.5%,the time from the initial sputtering to nodule forming was longer and degree of poisoning was lighter.In addition,nodules were not observed increasing for ITO:Nb/Ti target.The resistivity of ITO film rapidly increased from less than 5 × 10-4?·cm to 2 ×10-3?·m even bigger and the light transmittance decreased from more than 85%to less than 80%after poisoning.It was found that there were some hillocks on the surface of ITO film prepared after "poisoning" of target.And this is really the primary cause of the decline of the resistivity and the light transmittance of ITO film.Finally,the effects of substrate temperature,sputtering bias and film thickness on the properties of ITO:Nb/Ti film were studied systematically.The results show that the crystal structure of ITO:Nb/Ti film was relatively uniform,the surface roughness was the smallest.The lowest resistivity(1.2 × 10-4?)ITO:Nb/Ti film was obtained at the sputtering bias with 120V at room temperature.The transmittance of ITO:Nb films with different temperature,bias and thickness was above 87%,and for ITO:Ti films,it was more than 85%,up to 90%.ITO films with excellent photoelectric properties were obtained at room temperature for the first time due to its preferred orientation being(400)orientation.And this advantage could expand its application field by meeting the requirements of flexible substrate electronic devices and the silicon heterojunction solar cells.In this work,ITO:Nb/Ti films were prepared by sputtering single target doped with Nb/Ti for the first time.And also for the first time,the relationship between Nb/Ti oxide powder,ITO powder,ITO target material and performance to sputtering coating process and film performance was studied and analyzed systematically.The causes and improvement measures of target poisoning were described in detail.And the process parameters for optimizing ITO film preparation are determined.All these research achievement would make it possible that someday ITO target made in China would be used for TFT-LCD in large scale.