Research On Removal Mechanism Of Silicon Driven By Mechanochemical Synergic Energy

Nanomanufacturing is the commanding point of manufacturing technology competition in the 21^st century.It is the key technology to measure the level of national high-tech development and comprehensive strength.With the development of nanotechnology,the traditional surface processing technology has been unable to meet the requirements of practical applications(including telecommunications,national defense,aerospace,integrated circuit,optical manufacturing and other high-end equipments)in both high surface accuracy and nondestructive surface.Thus,the ultra-high precision surface manufacturing technology is urgently needed.Single crystal silicon is the most widely used semiconductor material,which also has the most mature researches and the most complete production lines in industry.Its surface processing accuracy requires sub-nanometer surface roughness,nanometer surface accuracy,and near zero subsurface damage layer.In ultra-high precision fabrication of silicon surface(such as nanocutting and nanopolishing),the surface material is removed in a ductile manner.Although no micro-crack can be found on the processing surface,there may still exist multi-damage layers in the subsurface,which may be composed of hydrolyzed layer(softened layer),impurity embedding(abrasive residue)and lattice distortion of subsurface layer.Due to the repeated action of multiple abrasive particles and the small contact area in the ultra-high precision machining of silicon surface,the rapid removal of material makes the actual reaction process unable to be real-time monitored in situ.In addition,due to the coupling effects of interfacial chemical reaction and external mechanical interaction,it is particularly difficult to characterize the atomic migration process and energy dissipation behavior in nanoscale.Current studies have not yet made a thorough understanding of the atomic migration mechanism and the evolution process of the surface/subsurface microstructure during the material removal on silicon surface.To reveal the surface/subsurface damage mechanism in the ultra-high precision processing and further enrich the theory of mechanochemical removal of silicon surface,the quantification model of mechanochemical removal and the energy dissipation mechanism in the atom migration process is established,and the method of atom-level nondestructive machined surface driven by mechanochemical synergic energy is proposed by studying the influences of reaction medium,tribopair materials and mechanochemistry on the material removal of silicon surface.This study has important guiding significance for further understanding the mechanism of mechanochemistry and optimizing the ultra-high precision machining with low subsurface damage and high efficiency on the silicon surface.It is expected that this theory can be extended to the ultra-high precision machining of other semiconductor and optical materials,and even indirectly promote the development of various high-tech industries such as the national electronic communications,biomedical,energy reserve,national defense and aerospace.The contents and innovations of this paper are summarized as follows:(1)The effect of surface oxide layer on surface wettability and water adsorption behavior was studied.A model for calculating the adhesion force considering the thickness and structure of the adsorbed water film was proposed.The influence mechanism of water layer structure on the tribological behavior(adhesion and friction)was revealed.The nanoasperity contact results have shown that the interfacial adhesion is not just controlled by water contact angle and the hydrogen bonding interactions of adsorbed water molecules,but also the structure of the surface oxide layer,and further affects the friction behaviors.When stored in the environment containing oxygen and water molecules,the hydrophobic silicon surface will undergo oxidation and hydrolysis reactions,resulting in the formation of an oxygen-containing layer and the surface hydrophilization.On hydrophobic surfaces(Si/H and Si_alcohol),the weakly hydrogen-bonded water layer on Si surface is dominated by liquid-like structure,and the adhesion force is mainly determined by capillary force and van der Waals force.On hydrophilic surfaces(Si_air and Si O_x/OH),strongly hydrogen-bonded water network grows gradually and the force for rupture of the solid-like water bridge transfers to dominate the adhesion force.By introducing a fitting parameter?,a solid-adsorbate-solid model with the consideration of the adsorbed water structure and oxide layer structure evolution is developed to illustrate how the surface oxide layer influences the interfacial adhesion of the nanoasperity contact.?value depends on the thickness and structure of the adsorbed water layer on substrate surface.(2)The effect of crystal structure on the mechanochemical and mechanical removal of silicon and its influencing mechanism were investigated.A quantification model of mechanochemical reaction on silicon surface was established by modifying the mechanical stress-assisted Arrhenius model combined with first-principles calculation results.The influence of external mechanical energy on mechanochemical reaction and the mechanochemical synergic energy mechanism were revealed.Both the mechanochemical removal rate and the shape of produced grooves on silicon surface is strongly affected by crystal structure.Different from mechanical removal,mechanochemical removal was not dependent on the crystallography dependent surface mechanical properties,but was mainly attributed to various atomic planar density and interplanar spacing in different crystal planes.The shape or facet of the mechanochemically-etched trench appears to be governed by the surface energy change,i.e.,thermodynamics.The material removal rate seems to be governed by the susceptibility to mechanical activation or assistance as well as the chemical activation energy.A quantitative model of mechanochemical reaction on the silicon surface was established.The mechanochemical energy coupling process in the material removal of Si/Si O₂ interface can be simply described as:mechanical energy promotes the water-assisted hydrolysis reactions of Si-Si bonds by reducing the reaction activation energy and providing energy to reach the activation state,and ultimately realizes the stripping of silicon atoms on the substrate.The quantification model fitted the mechanochemical reaction rate under different shear stress by the mechanical energy-assisted modified Arrhenius formula,obtained the activation volume of the interfacial mechanochemical reaction,which provided further understanding in the influencing mechanism of the external mechanical energy on mechanochemical reactions.(3)The effects of the structure of mechanical damage layer and the chemical activity of counter-surface on the mechanochemical removal of silicon surface were elucidated,and the influencing mechanisms were revealed.The mechanical damage layer on the substrate surface significantly affects the interfacial mechanochemical reaction.The mechanochemical reactions are suppressed on hillock structure,but facilitated on groove structure.The analysis indicated that the influencing mechanism is mainly attributed to the topmost oxide layer with different thickness,which could effectively inhibit the interfacial mechanochemical reaction and reduce the material removal efficiency.Furthermore,the counter-surface chemistry dependent mechanochemical removal on silicon surface indicates that the chemical activity of counter-surface affects the mechanochemical reactions via altering the activation thermal energy for the formation of bonding bridge,but has little influence on decreasing the activation energy barrier in the rupture of Si-Si bonds of silicon substrate.(4)The critical contact pressure and critical dissipated energy for mechanochemical and mechanical removal on the silicon surface were further determined.The energy dissipation pathway and its proportion during the material removal process on silicon surface were clarified,and the mapping relationship between surface atom migration and dissipated energy was established.Finally,according to the thermal activation theory and the interfacial mechanochemical bonding mechanism,the atom-level nondestructive processing method of silicon surface driven by mechanochemical energy was proposed.The decrease in critical dissipated energy by water content and sliding cycle can be explained as the facilitative mechanochemical reaction(mianly includes the formation of more Si-O-Si bonding bridges and rupture of more Si-Si networks)and the residual energy in the scratched area,respectively.Mechanochemical removal consumes only 0.4%of critical dissipated energy and 5.6%of dissipated energy of mechanical removal,which is attributed to the reduced energy barrier of Si-Si bond breakage and no lattice distortion underneath the machined area.Mechanochemical removal achieves nearly zero subsurface damage whereas mechanical removal consumes 31%of energy in severe amorphization and deformation underneath.Through coupling the multiple sources of energy from mechanical and chemical interactions,the huge energy barrier of bulk material removal is decomposed into countless small removal steps and the reaction activation energy is effectively decreased,which improves the energy utilization efficiency and reduces the energy consumption significantly.