Study On Stress-induced Silicon Phase Transition Based On Nanoindentation And Nanoscratch

With the rapid development of science and technology,micro electromechanical system(MEMS)has been widely used in biology,medicine,environmental protection,aerospace,agriculture,industry and military fields.Single-crystal silicon has many advantages,such as dopability,mature preparation technology,reliable mechanical properties,high thermal conductivity and anti-static ability,and is one of the important materials for the preparation of mems.However,the phase transformation of single-crystal silicon is easy to occur under external forces,and results in its own performance change,which directly affects the stability of MEMS.At the same time,the service conditions of devices often involve pressure,friction and high temperature,making the phase transformation behavior of single-crystal silicon more complex.Therefore,this paper takes single-crystal silicon(100)as the research object,and systematically studies its phase transition behavior under different indentation,scratch and temperature conditions with the help of instrumented indentation testing,and the effect of TaN film of different thickness on the phase transition of silicon has been also discussed furtherly.First of all,the effect of loading/unloading rate and ambient temperature(room temperature,50 o C and 100 o C)on the phase transition mechanism of silicon has been studied by nanoindentation.The results indicate that when the unloading rate is certain(1mN/s),the probability of pop-out phenomenon increases with the loading rate increases from 0.25mN/s to 2.5mN/s,it means that Si-II can be converted into Si-III/Si-XII more easily,the main reason is that the loading rate is larger,Si-I will be more easily transformed into Si-II under the same loading depth and unloading rate,and promotes the occurrence of pop-out phenomenon.While the unloading rate increases form 0.25mN/s to2.5mN/s,the probability of pop-out phenomenon will decrease,and it is independent of the loading rate,because the transformation from Si-II to Si-III/Si-XII is a nucleation and growth process,and the unloading rate is lower,there will be more time to provide for nucleation while maintaining the pressure,which is conducive to the phase transition.Meanwhile,it can be concluded that,unloading rate plays a more dominant role in the probability of pop-out.With the increase of ambient temperature,the probability of pop-out is more likely to decrease,the temperature promotes the formation and growth of new phase,and the hardness and effective elastic modulus of material decreases too.Secondly,the phase distribution of silicon in the scratch was studied by nanoscratch,and then systematically investigated the effects of scratch speed,loading rate,scratch load and ambient temperature on the phase transition of silicon.The results show that when the scratch load is certain(150mN),and scratch speed decreases from 2?m/s to 0.5?m/s,although themaximum scratch depth is close,the residual scratch depth,width and total pile-up are smaller under the lower scratch speed,and the proportion of Si-III and Si-XII increases,because of the slower scratch speed will provide more time for the nucleation and growth of the new phase,which is conducive to the phase transition,indicating that the phase transition is conducive to the elastic recovery of the scratch.While when the scratch speed is certain(1?m/s),the proportion of Si-III and Si-XII increases with the maximum scratch load increases from 100 mN to 200 mN,it means that the phase transition degree is strongly affected by the scratch load,and the residual scratch depth,width and total pile-up also increase.With the increase of temperature,the phase transition in the nanoscratch is more likely to occur,and the residual scratch depth,width and total pile-up increase,because of the decrease of hardness and elastic modulus.Finally,TaN/Si was taken as the object in research,the effects of different thicknesses film on the phase transition of silicon were studied by the nanoindentation and nanoscratch.It is found that TaN film has an obvious protective effect on single-crystal silicon,and the hardness and scratch resistance increase with the increase of thickness.TaN also shows an inhibitory effect on the pop-out phenomenon during the nanoindentation,and the inhibitory effect became more obvious with the increase of thickness.