Gold Wire Bonding Reliability In 3D NAND Flash

With the explosive growth of intelligent terminals,cloud computing and artificial intelligence technology,people have an increasing demand for semiconductor memory.3D NAND breaks through the scaling limitation of traditional NAND Flash through three-dimensional stacked memory cells and becomes the mainstream product of next generation memory.The emergence of 3D NAND has led to the flourishing development of downstream packaging industry.At present,most of the NAND packaging uses threedimensional stacking.Wire bonding is still the mainstream technology to realize 3D NAND stacking packaging because of its low cost and wide application.However,due to the late start of NAND Flash industry in China,the effects of 3D NAND stereo structure on bonding process parameters and the reliability problems caused by thinner pads are not fully understood.1.The quality of bonding process determines the stable electrical and mechanical connection between the chip and the outside world.In the research of bonding process,the mechanical properties of traditional NAND and 3D NAND chip pads are characterized by nanoindentation method.Because of its thick circuit layer structure,the hardness load curve of 3D NAND pads will lag behind that of planar NAND,and the overall hardness will be smaller than that of planar NAND.On this basis,the bonding process of the first solder joint is systematically studied through DOE experimental design.The effects of input parameters such as ultrasonic power,C/V value and bonding temperature on the output parameters such as weld ball size,weld ball thickness,tensile strength and shear strength were investigated.The bonding interface was separated by chemical etching,and the area of intermetallic compounds and the morphology of the bonding interface were observed.Finally,it is found that the increase of initial bonding force and bonding time is helpful to improve bonding strength.2.The intermetallic compounds at the bonding interface have an important influence on the bonding strength of solder joints.In the study of intermetallic compounds at bonding interface,a series of characterizations were carried out on the bonded samples.It was found that three layers of intermetallic compounds were formed at the bonding interface,namely Au4 Al,Au8Al3 and Au Al2.Au8Al3 occupies the main body of IMC.At the same time,holes were found between Au4 Al and welding balls.In the 24 h annealing experiment,it was found that the IMC area of the bottom bonding interface was increasing,the thickness of IMC was also increasing,the thickness of three-layer IMC Au8Al3 was the most obvious,and the other two layers were slightly thicker.Holes were also found between Au8Al3 and Au4 Al.Compared with traditional thick pads,the IMC growth rate of 3D NAND pads with thinner aluminium layer is slower,because the insufficient number of aluminium atoms in thin pads inhibits the diffusion of aluminium.Finally,the order of IMC formation is discussed from thermodynamics and kinetics:Au8Al3,as the phase with the largest negative enthalpy of formation,is preferentially generated in three IMCs;however,the thickness of Au8Al3 layer limits the diffusion of Au and Al,so Au4 Al and Au Al2 are formed on both sides by reactants.3.The service conditions and life of products are the most concerned of users,and improving the life of devices is the goal of packaging reliability research.In the study of reliability,the UHAST(unbiased high accelerated stress test)experiment was used to test the reliability of bonding points systematically,and the failure mechanism was studied through a series of characterizations.Transmission electron microscopy(TEM)showed that the interface between welding ball and IMC was seriously corroded.Combining EDS and selected area electron diffraction(SED),Au4 Al phase and Al2O3 corrosion product were identified.From the thermodynamic point of view,Au4 Al is more unstable than Au8Al3.At the same time,Kirkendall voids are easily concentrated on both sides of Au4 Al during IMC growth,which provides a channel for water,gas and external impurities,and ultimately leads to corrosion.Because of the slow growth of IMC in thin pad,the thicker IMC layer can not be formed by short annealing time.By increasing annealing time,the thicker IMC layer is mainly formed as(Au8Al3)as the barrier layer of Al diffusion,which slows down the formation of Au4 Al and achieves the purpose of improving the high temperature and humidity reliability of bonding interface.