Reflow and agglomeration characteristics of Cu thin films in variousannealing, ambients, and effects of the grain growth and the residualstress of the films on reflow were investigated. Reflow can make metalatoms move to fill up the preformed micro-patterns upon post-annealing,which is one of the me...
Reflow and agglomeration characteristics of Cu thin films in variousannealing, ambients, and effects of the grain growth and the residualstress of the films on reflow were investigated. Reflow can make metalatoms move to fill up the preformed micro-patterns upon post-annealing,which is one of the methods to fill hole and trench patterns with metalin fabricating metal interconnects for integrated circuits(ICs). Reflow isconsidered as a filling method for Cu interconnects in future ICs becauseof its simplicity, but high temperature for processing is the demerit ofreflow. Gradients of the chemical potential proportional to the curvatureof metal surface are the driving force for atom movement, and amongvarious transport mechanisms the surface diffusion is dominant for thereflow with micro-patterns. While the self-diffusion of Cu along thesurface might be important in reflow, this work showed that theagglomeration and the grain growth of Cu films driven by intrinsic stressalso gave critical effects on reflow phenomena at elevated temperature. Multi-layer samples with 150nm or 400nm thick Cu/TiN/Ti/SiO2/p-type Sistructures were annealed in nitrogen, helium, argon, or oxygen, thencharacterized by SEM, TEM, XRD, and AES. For measuring the thicknessof Cu oxide and the residual stress of Cu films, coulometric reductionmethod and sine square psi method were used, respectively. Theagglomeration of Cu films occurred during the annealing in an oxygenambient, while it was not observed in inert ambients. The trench patternsof 0.8um with aspect ratio of 1.3 were completely filled at annealingtemperature higher than 400 degrees C due to the preferential agglomerationinside the patterns in an oxygen ambient. With increasing annealingtemperature, the quantity of Cu oxide increased by oxidation reaction andthe resistivity of Cu films rose drastically due to the poor connectivity ofCu grains resulted from the agglomeration. While the thickness of Cu oxideincreased with annealing time at 400 degrees C in an oxygen ambient,the agglomeration of Cu films terminated within 5min with 18.3nm thick Cuoxide on the Cu surface. The Cu oxide formed could be removed bypost-annealing in an H2(10%)/Ar ambient for 25min so that the thin oxidedid not matter for the use of this reflow process. The normal grain growth,which resulted in increased thermal instability of Cu films, was observedupon oxygen annealing. If the ratio of film thickness to grain size is small,the thermal groove will be able to reach the underlayer, which will resultin the decomposition and the agglomeration of the film. Because of thelarger volume of Cu oxide than that of Cu, the formation of Cu oxide ledto the increase of compressive stress. Since the area of grain boundariesreduces during the grain growth, tensile stress is produced by reductionof the amount of Cu oxide located at grain boundaries. As a result,only compressive stress can be relaxed by grain growth. It was shown bycalculation that the compressive stress could decrease from 5.6GPa to 1.6GPadue to the grain growth upon oxygen annealing. The same result ofthe grain growth and the agglomeration upon oxygen annealing was obtainedin case of Ag thin films. In conclusion, it seems that the agglomeration andthe reflow in an oxygen ambient are the phenomena resulted from therelaxation of compressive stress via the grain growth.
Reflow and agglomeration characteristics of Cu thin films in variousannealing, ambients, and effects of the grain growth and the residualstress of the films on reflow were investigated. Reflow can make metalatoms move to fill up the preformed micro-patterns upon post-annealing,which is one of the methods to fill hole and trench patterns with metalin fabricating metal interconnects for integrated circuits(ICs). Reflow isconsidered as a filling method for Cu interconnects in future ICs becauseof its simplicity, but high temperature for processing is the demerit ofreflow. Gradients of the chemical potential proportional to the curvatureof metal surface are the driving force for atom movement, and amongvarious transport mechanisms the surface diffusion is dominant for thereflow with micro-patterns. While the self-diffusion of Cu along thesurface might be important in reflow, this work showed that theagglomeration and the grain growth of Cu films driven by intrinsic stressalso gave critical effects on reflow phenomena at elevated temperature. Multi-layer samples with 150nm or 400nm thick Cu/TiN/Ti/SiO2/p-type Sistructures were annealed in nitrogen, helium, argon, or oxygen, thencharacterized by SEM, TEM, XRD, and AES. For measuring the thicknessof Cu oxide and the residual stress of Cu films, coulometric reductionmethod and sine square psi method were used, respectively. Theagglomeration of Cu films occurred during the annealing in an oxygenambient, while it was not observed in inert ambients. The trench patternsof 0.8um with aspect ratio of 1.3 were completely filled at annealingtemperature higher than 400 degrees C due to the preferential agglomerationinside the patterns in an oxygen ambient. With increasing annealingtemperature, the quantity of Cu oxide increased by oxidation reaction andthe resistivity of Cu films rose drastically due to the poor connectivity ofCu grains resulted from the agglomeration. While the thickness of Cu oxideincreased with annealing time at 400 degrees C in an oxygen ambient,the agglomeration of Cu films terminated within 5min with 18.3nm thick Cuoxide on the Cu surface. The Cu oxide formed could be removed bypost-annealing in an H2(10%)/Ar ambient for 25min so that the thin oxidedid not matter for the use of this reflow process. The normal grain growth,which resulted in increased thermal instability of Cu films, was observedupon oxygen annealing. If the ratio of film thickness to grain size is small,the thermal groove will be able to reach the underlayer, which will resultin the decomposition and the agglomeration of the film. Because of thelarger volume of Cu oxide than that of Cu, the formation of Cu oxide ledto the increase of compressive stress. Since the area of grain boundariesreduces during the grain growth, tensile stress is produced by reductionof the amount of Cu oxide located at grain boundaries. As a result,only compressive stress can be relaxed by grain growth. It was shown bycalculation that the compressive stress could decrease from 5.6GPa to 1.6GPadue to the grain growth upon oxygen annealing. The same result ofthe grain growth and the agglomeration upon oxygen annealing was obtainedin case of Ag thin films. In conclusion, it seems that the agglomeration andthe reflow in an oxygen ambient are the phenomena resulted from therelaxation of compressive stress via the grain growth.
주제어
#Cu Reflow Agglomeration Oxidation Grain growth 구리 배선 응집 산화 입자성장
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