Study On Wear Behavior And Mechanism Of Silicon-based Materials At Nanoscale

Silicon-based materials(e.g.,silicon,silica,etc.)have been widely used for the manufacturing of micro/nanoelectromechanical systems(i.e.,MEMS/NEMS).However,due to its poor tribological properties as well as the size and surface effects of materials on the micro/nanoscale,the nanowear problems widely exist in the silicon-based MEMS/NEMS devices with physically moving,colliding,and rubbing components,and thus the reliability and lifetime of silicon-based MEMS/NEMS face significant challenges and limitations.Monocrystalline silicon as the most typical semiconductor material,has been widely used in nanotechnology(e.g.,MEMS/NEMS,large scale integrated circuits,biochips,etc.)because of its excellent lithographic,physical and mechanical properties.Chemical mechanical polishing(CMP)is the indispensable key technology in nano and even atomic scale precision surface manufacturing of silicon-based nanotechnologies.The silicon CMP is actually a nanowear problem of silicon-based materials,but material removal mechanism at the micro/nanoscale during CMP is less known and needs further exploration.The nanowear of silicon-based materials is not only a key problem for the applications of nanotechnology,but also a fundamental problem for the nanomanufacturing of silicon-based materials.Therefore,the results of this paper will not only provide a reference for the protection and design of silicon-based MEMS/NEMS,but also promote the development of silicon CMP technology.In this paper,molecular dynamics(MD)simulations with reactive force field(ReaxFF)were carried out to quantitatively study the tribological behaviors and tribochemical reactions between silicon-based materials,and to reveal the wear mechanisms of silicon-based materials at atomic scale.The main contents and results of the thesis can be summarized as follows:Firstly,ReaxFF-MD simulations were performed to explore the tribological behavior between fully hydroxylated amorphous silica(a-Si O₂)surfaces as a function of surface silanol density.The results show that the interfacial friction and wear are greatly reduced by increasing surface silanol density,which originates from the suppression of the initial formation of interfacial Si-O-Si bridge bonds.Two different tribochemical reactions resulting in the formation of interfacial Si-O-Si bridge bonds are observed,i.e.,one occurring between two silanol groups,which is insensitive to changes in silanol density,and the other occurring between a silanol group and a surface Si-O-Si bond,which is strongly suppressed with the increase of silanol density.We decouple the contributions of these two Si-O-Si bond formation mechanisms to the observed tribological behavior,and find that the latter formation mechanism plays a dominant role.Furthermore,the changes in the geometry and structure of fully hydroxylated a-Si O₂ surface caused by the increased surface silanol groups also play an important role in the tribochemical reactions and the tribological performance of the a-Si O₂/a-Si O₂system.This study provides a deeper insight into the effect of surface silanol groups on the tribological behaviors of silicon-based materials.Secondly,ReaxFF-MD simulations were used to find the atomic-level wear mechanisms occurring at the sliding interface of fully hydroxylated?-quartz(010)surfaces as a function of interfacial water amount.The results show that there are two kinds of rupture behaviors of surface siloxane bonds.One is resulted from only hydrolysis reaction between surface siloxane bonds and water.The other is also caused by the hydrolysis reaction,but is assisted by the interfacial siloxane bonds.Both rupture behaviors can directly lead to tribochemical wear on silica surfaces.In the presence of less than a full monolayer water,the degree of wear induced only by hydrolysis reaction is affected by the change of interfacial shearing actions caused by increasing water molecules.However,the degree of wear assisted by interfacial siloxane bonds is related to the dual role of interfacial water in interfacial siloxane bonds.It is also noted that the load-dependent probability that the occurrence of tribochemical wear is caused by the formation of an interfacial siloxane bond ranges from 18%to 37%.In addition,mechanical wear,characterized by the local lattice distortion at subsurface,is also observed in our simulations.This study shows that even though no interfacial siloxane bond is formed,tribochemical wear of silica can still occur due to stress corrosion,and provides further insights into the wear mechanism of silica.Then,the CMP processes on a Si(110)surface polished with an a-Si O₂ particle were investigated using ReaxFF-MD simulations.We elucidate the oxidation effect of H₂O on the Si surface before the CMP process.The surface-adsorbed O atoms dissociated from the surface-adsorbed OH groups exist in the form of Si-O-dangling bonds and Si-O-Si bonds.During CMP at the Si(110)/a-Si O₂ interface,the insertion of O atoms into the surface,the formation of interfacial bridge bonds,the adsorption of H atoms on the surface four-coordinated Si atoms,and pure mechanical shear can induce the breaking of the bonds on the oxidized-Si(110)surface,resulting in the occurrence of wear on the surface.We decouple the contributions of these wear mechanisms to the wear degree of the surface,and find that the first two mechanisms play a decisive role;the wear of the surface is the coupled results of tribochemical reaction and mechanical interaction,but the tribochemical reaction plays a dominant role;the wear of the surface is the combined results of multiple atomic-level wear mechanisms.We track the source of the O atoms involved in surface and interfacial Si-O-Si bonds,and find that the main source of the O atoms of surface Si-O-Si bonds is the water oxidized Si(110)surface,and that of interfacial Si-O-Si bridge bonds is the fully hydroxylated a-Si O₂surface.The result suggests that the presence of oxide layer can facilitate the wear of Si(110)surface mainly by providing O atoms to insert into the surface,rather than by providing additional reaction pathways to form interfacial Si-O-Si bridge bonds.This study reveals the important role of the oxidation of Si surface in enhancing Si material removal during CMP.