Effect Of SiO₂ Structure On The Material Removal Of Silicon At Micro-Scale

Due to its excellent physical and mechanical properties,monocrystalline silicon has been widely used in microelectronics integrated circuit(IC).Currently,silicon basal diameter in IC is increasing while the line width is decreasing gradually.The line pith could be reduced to 14 nm,and it will continue to be decreased to less than 10 nm in the near future.In order to maximize the integration density of the silicon base,it is needed to reduce the roughness of silicon to the sub-nanometer scale,and minimize the residual surface and subsurface damage.Currently,chemical mechanical polishing(CMP)is recognized as the optimal choice to obtain such perfect surfaces by removing the extra material.However,material removal in traditional CMP process is mainly attributed to the synergy of mechanical interaction and chemical reaction.Serious surface damage could be produced on silicon substrate during the traditional CMP process,such as dislocation,micro cracks,chemical etching fog spot and other mechanical damage,which greatly reduce the quality and efficiency of polishing process.In recent years,the tribochemical mechanism of CMP process has received wide attention.However,previous reports mainly focused on the influence of load,speed and atmosphere on nanowear behavior of monocrystalline silicon,and the study on the effect of SiO2 abrasive structure on the material removal of monocrystalline silicon is still lacking.In view of the above situations,this thesis focused on the material removal of monocrystalline silicon by non-porous and micro-porous SiO2 spheres at microscale.After microwear tests,the 3D surface morphologies of the wear scars were scanned by white light interferometer and the chemical structure and composition of the worn area were characterized by Energy Dispersive X-ray(EDX)and Raman spectrum analysis.We also detected the related influencing factors on the tribochemical wear and gave a deeper understanding on the tribochemical wear mechanism of monocrystalline silicon.The main research contents and innovations are presented as follows:(1)The wear of original silicon against non-porous and micro-porous SiO2 spheres was investigated comparatively in humid air and in pure water.Under the same sliding condition,non-porous SiO2 spheres caused more serious wear on silicon substrate than that by micro-porous SiO2 sphere in humid air.However,as a contrary result,micro-porous SiO2 sphere induced more material removal in pure water.The occurrence of silicon wear depended on water content and micropore structure of SiO2 sphere.The former associated the hydrolysis reaction and the latter improved the reaction rate,both of which promoted the tribochemical reaction on silicon surface.(2)The native oxide layer of monocrystalline silicon has significant effect on material wear.On one hand,the surface of original silicon(with native oxide layer)is hydrophilic with hydroxyl,and hydrophobic silicon(without oxide layer)surface is terminated by hydrogen bonding,so the original silicon surface in humid air would absorb more water molecules,resulting in more serious tribochemical wear.On the other hand,since the number of water molecules maybe independent with the surface structure in water environment,the effect of the Si-O bond in native oxide layer is more obvious.As the chemical reaction barrier of Si-O bond is larger than Si-Si bond,so the tribochemical wear on original silicon was lighter than that on hydrophobic silicon surface in pure water environment.(3)The surface damage of monocrystalline silicon in nitrogen environment is mainly attributed to the tribochemical wear with water molecules,and the relationship between the wear volume and contact pressure of monocrystalline silicon follows the Arrhenius formula in chemical reaction.The occurrence of tribochemical reaction between Si/SiO2 pair is inseparable from mechanical friction,and the number of water molecules has more significant effect on the tribochemical wear of monocrystalline silicon.On one hand,the micropores can increase the contact area and exacerbate the tribochemical reaction between the contact interfaces under the same contact pressure.On the other hand,the micropores can also store water molecules which increases the tribochemical wear at the same time.The Raman and EDX analysis of wear debris under different conditions showed that the Si-OH bond existed in the wear products,which proved the existence of hydrolysis.In addition,it was also found that the micropores could promote the occurrence of the tribochemical reaction,and the intension of Si-OH bond was stronger in this condition.In conclusion,the tribochemical wear of monocrystalline silicon was closely related to contact mode,environmental moisture and the oxide layer on the substrate surface.Meanwhile,the micro structure of SiO2 sphere increased the contact area and water amount of the sliding interface which promoted the tribochemical wear of monocrystalline silicon.In addition,the tribochemical reaction at the interface of Si/SiO2 pair could be promoted in humid air and limited in pure water by native oxide layer on the silicon surface.The above results may provide experimental and theoretical support for the improvement in CMP process of monocrystalline silicon,and also enrich the basic theory of micro and nano tribology.