Research On Nanocrystallization Process And Mechanism Of High Purity Tungsten Powder

Tungsten has the advantages of high melting point,anti-electromigration and high thermal stability.It is used in semiconductor devices in the form of thin films,especially in the manufacture of advanced semiconductor memory chips.Tungsten sputtering target is used for preparing tungsten thin films with uniform thickness and low resistivity and its properties depend on the properties of high-purity tungsten powder.High-purity tungsten sputtering targets require tungsten powder with high purity(5N)and small particle size(nano-level).In this paper,high-purity ammonium paratungstate is used as the raw material,and a combination of precipitation method and two-stage hydrogen reduction method is designed to prepare high-purity nano-level tungsten powder.The main research results are as follows:(1)The influence of the feeding sequence on the morphology of WO3 powder was explored.The white ammonium paratungstate powder with higher dispersion was obtained as the precursor by the adding concentrated HCl to ammonium tungstate solution,and the WO3 powder obtained after calcination was dispersed well.The precursor obtained by adding ammonium tungstate solution to concentrated HCl is yellow colloidal WO3·0.33H2O,and the WO3 powder obtained after calcination has the phenomenon of hard agglomeration.Concentrated HCl was used to adjust the pH of the ammonium tungstate solution to 0.5,1,2,3,and 4 respectively.When the pH was 1,the average particle size of WO3 powder was 45 nm.TG-DSC thermal analysis was performed on the ammonium paratungstate precipitation.The whole decomposition process is divided into 5 stages.When the calcination temperature is higher than 520?,the WO3 transformation is completed.When the calcination temperature is 700?,the average particle size of WO3 powder is 130 nm.GDMS purity analysis was carried out on the WO3 powder,the total content of impurity elements involved in the 5N standard does not exceed 5 ppm.(2)Using the above-mentioned WO3 powder as a raw material,keeping the reduction time of each stage for 150 min,the reduction temperature of each stage was explored separately.With the increase of the reduction temperature in the first stage,the particle size of the tungsten powder increases.When the temperature in the first stage was 600?,the average particle size of the tungsten powder was about 1.1 ?m.When the second stage of reduction temperature was set to 800,900? respectively,WO3 was reduced to tungsten powder completely.When the second stage reduction temperature was 800?,the particle size of 930 nm was smaller than that of second stage temperature of 900?.Tungsten powder was prepared with a heating gradient with a temperature of 600? in the first stage with holding temperature for 150 min,and temperature of 800? in the second stage with holding temperature for 150 min.The reduction path was WO3?WO2.9?WO2?W.When the thickness of the sample layer was reduced from 20 mm to 2.5 mm,the particle size was reduced from 930 nm to 330 nm.Increasing the hydrogen flow rate takes away more water vapor in reduction system,which can reduce the particle size of the tungsten powder.When the hydrogen flow rate was increased from 200 mL/min to 500 mL/min,the particle size of the tungsten powder decreased from 330 nm to 210 nm.The tungsten powder obtained was assayed by GDMS.The impurity elements content is less than 2 ppm.The tungsten powder basically meets the application requirements of 5N nano-tungsten powder.