Molecular Dynamics Simulation Study On The Subsurface Cracks Of Monocrystalline Silicon

Monocrystalline silicon is widely used in the manufacture of semiconductor devices such as integrated circuits,optoelectronic devices and micro-sensors.Ultra-precision processing technology of silicon is an important symbol for measuring the advanced manufacturing level of the country.However,silicon is a typical hard and brittle material so subsurface cracks are easily induced in the machining process of silicon.The cracks will significantly affect the mechanical strength and surface integrity.Accordingly,research on the initiation and propagation of subsurface cracks to improve the reliability and life of devices is of great significance for accelerating the development of the advanced semiconductor manufacturing industry.But it is difficult to perform the in-situ characterization of the fracture process of silicon at nanoscale.At the same time,traditional simulation techniques such as the finite element method cannot study the origin and evolution of cracks from the atomic point of view.Consequently,the subsurface damage mechanism of silicon has not been revealed clearly.Therefore,in this work,the subsurface damage evolution during nano-cutting of single crystal silicon is deeply studied based on molecular dynamics(MD)simulation.The subsurface crack initiation mechanism is revealed through crystal structure analysis,stress distribution,atomic displacement tracking and other methods.The main work is as follows:Cracks did not appear in the past MD studies because the undeformed chip thickness was too small.An improved nano-cutting model was used to conduct large-scale MD simulations,so as to directly observe the transformation of monocrystalline silicon from ductile processing to brittle processing.Different forms of subsurface damage during nano-cutting were studied through crystal structure identification and radial distribution function.When the tensile stress level exceeds the fracture limit of the material,dissociation failure will occur at the location of stress concentration.In addition,the effects of different machining parameters on the subsurface damage and material removal mode during nano-cutting were explored,and the variation trend of subsurface damage with machining parameters was reasonably explained by combining phase transition and dislocation.Through the analysis of atomic density and displacement tracking during the cutting process,the phenomenon of high-pressure-phase-transformation induced densification and plastic flow of silicon is obtained.The subsurface crack initiation mechanism is proposed from the atomic point of view,which emphasizes that the transformed flow plays an important role in crack nucleation.In order to study the phenomenon of phase-change atoms filling the crack cavity,uniaxial tensile simulation was used to prove its recovery effect on the mechanical properties of silicon.The semi-ductile processing of brittle materials through selfhealing cracks was proposed.A single-point diamond tip scratching experiment of the silicon wafer was carried out.The scratch surface quality and subsurface damage microstructure at different scratching depths were studied by scanning electron microscopy,transmission electron microscopy and other characterization methods.The material removal mode transition and brittle-ductile transition process during scratching were studied using the onset of chips and the crack.the reliability of the MD simulation results is verified by comparing the subsurface damage structure in the experimental and simulation results.