Study On Mechanism Of Ultra-fast Laser Induced Surface Modification Of Silicon Carbide

Silicon carbide is an excellent third generation of semiconductor material which has a number of advantages,including wide band-gap,large drift velocity,high breakdown voltage,high thermal conductivity and high temperature resistance,often as the substrate material of high frequency and high power devices,used in electronic communications,industrial equipment,aerospace and other fields.As an excellent substrate,silicon carbide needs ultra-precision machining on the wafer surface to make the roughness less than 0.5nm.At present,the ultra-precision processing of silicon carbide wafer mainly applies grinding and chemical mechanical polishing(CMP)processes,and there are also some secondary damage and low efficiency problems,resulting in high manufacturing cost of silicon carbide substrates.Industrial ultrafast laser has extremely high peak power density(>10¹⁴W/cm²),very short pulse width(dozens fs?10ps)and extremely high repetition rate(?MHz),which also determine the characteristics of no material damage,small heat-affected area and so on.If a modified layer,that can be easily polished,can be formed on the surface of silicon carbide by ultrafast laser,it can provide a modified substrate for subsequent CMP which has no secondary damage and is easy to polish,so this way can replace the grinding process and solving the problem of low polishing efficiency.Therefore,this paper proposes a new process combining ultrafast laser-induced silicon carbide surface modification and chemical mechanical polishing to improve the silicon carbide polishing efficiency,by studying the mechanism and technology of ultrafast laser inducing silicon carbide surface modification.The main research methods and conclusions are as follows:(1)In order to study the effect of fs-laser and silicon carbide,femtosecond laser was used to irradiate silicon carbide wafers with single-pulse and multi-pulse method respectively,the changes in morphology,structure and composition of the material surface at different energy and pulse numbers were analyzed.The results show that no periodic structure will form when the surface is irradiated by just a laser pulse,and only gasification and nucleation of material and plasma will occur,which will come back to the surface and condense into nanospheres and sheet-like structures after cooling.The ablation area gradually extends from center to edge of the spot.However,the periodic ripple structure will form on the surface when the surface is irradiated by multiple laser pulses.For the formation of periodic ripple structure,the more pulses are,the smaller energy will be required,and the greater energy is,the smaller number of pulses will be required.In addition,the ripples showed two different states at the center and edge of the laser spot.Generally,the period of center ripple is 200nm?250nm and the edge ripple is80nm,which are also named low-frequency ripple and high-frequency ripple.Oxygen can be detected in both areas,and the oxygen content is higher in the high-frequency ripple area.When the pulse number is 50 and the incident energy reaches 1.23J/cm²,the central ripple begins to gradually disappear and melt into the hole structure,and the diameter and depth of the hole will increase as the energy increases.When the equivalent pulse number is 10?30 and the incident energy reaches 1.23J/cm²,a relatively uniform and flat structured surface can be gained at the spot area.(2)According to the effect of femtosecond laser and silicon carbide,the appropriate parameter window was selected to complete the large-area surface modification by scanning and overlapping,and the influence on the surface modification was investigated by adjusting the laser parameters and processing environment.The results show that the surface roughness of the modified surface can be optimized to a certain extent by adjusting the laser parameters.Generally,the surface roughness will decrease when the scanning speed increases or the energy reduces,but the material removal amount at this time is less,which is not conducive to remove the sub-surface defects of silicon carbide.However,by the same parameters,silicon carbide processing in a vacuum and oxygen environment can get a lower surface roughness and larger material removal amount,and the periodic ripples formed on the surface have a better continuity.(3)Chemical mechanical polishing was used as an assessment tool and the polishing efficiency and quality of polished surface were standards for evaluating.The above-described modified surfaces were taken in chemical mechanical polishing to analysis the effect on polish.The results show that the removal rate of the laser-modified surface was increased from 0.62?m/h to 1.73?m/h compared to the original silicon carbide cutting wafer,so the polishing efficiency was improved by about 3 times.At the same time,the defects on the polished surface also reduced significantly.However,not all modified surfaces have a gain in CMP,some fluctuating surfaces with micro-pillars and micro-groove structures may also increase the burden of CMP.In general,the modified surface with uniform periodic ripple structure can promote CMP well.The smoother the modified surface is,the better surface quality would be gain after CMP,and the surface with irregularity degree more than 2?m will still retain some fluctuation after CMP,which is not conducive to the overall flattening of silicon carbide.