| Silicon Carbide?SiC?is a third-generation semiconductor material,also known as a wide-bandgap semiconductor material.Compared with the first generation semiconductor material silicon?Si?and the second generation semiconductor material gallium arsenide?GaAs?,SiC has significant performance advantages,such as high temperature resistance,radiation resistance,applied in high voltage and high frequency condition.It is therefore an ideal substrate material for optoelectronic components that operate at extreme environmental or high power levels.However,its high hardness,brittleness and strong chemical inertness make it difficult to thin and smooth the SiC wafer.On the one hand,the material itself has poor process performance;on the other hand,the process requirements of the substrate wafer are very high,because as an essential component of integrated circuit fabrication,the epitaxial layer requires the substrate surface to be flat and smooth,and as free as possible from defects and damage.At the same time,the functions and applications of integrated circuit chips have been expanded under the impetus of technologies such as intelligence and Internet of Things,while these functional requirements of high-end chips will inevitably make the thickness and surface properties of the substrate wafer develop towards thinning,smoothing and free of damage.For example,artificial intelligence?AI?chips need to achieve deep learning and strong perception ability,which leads to higher requirements in terms of power consumption,reliability and size.Thus,the characteristic size and integration of integrated circuits approach to the physical limit,because chip thinning can reduce internal resistance,improve chip heat dissipation performance,enhance circuit stability,and reduce chip volume.Apart from that,MEMS devices,wearable intelligent equipment and so on require the chip to have the characteristics of miniaturization and thinning.In order to deal with the process difficulty of thinning and smoothing wafer caused by high hardness,brittleness and strong chemical inertia,both the free abrasive lapping and chemical mechanical polishing for single crystal SiC wafer assisted by ultrasonic vibration were studied in this paper.The material removal mechanism and surface generation characteristics of SiC wafer under the above two processes were revealed by theoretical analysis and numerical simulation,simultaneously the experimental device was designed and manufactured,on the basis of which the related experiments were carried out and the test results were analyzed.Specifically,the main research contents are as follows:?1?The mechanical response and mechanism of brittle materials in pressing and scratching were analyzed.On this basis,the single abrasive pressing and scratching for single crystal SiC was simulated by SPH coupling FE in ls-dyna.Three main mechanical states of abrasive under conventional free abrasive lapping were analyzed:two-body wear,three-body wear and free non-action state.Therefore,the dynamic changes of three kinds of mechanical states of abrasive under the ultrasonic assistance were obtained:the two-body wear abrasive combines the original horizontal motion with normal harmonic motion and periodically acts on the specimen surface,the free non-acting abrasive is activated and randomly impacts the specimen surface,and the three-body wear abrasive impacts or hammers the specimen surface.For the two-body wear of abrasive,the scratching for single crystal SiC under different rates and depths were simulated respectively,and for the mechanical effect of abrasive under the ultrasonic assistance,pressing simulation under different impact rates,impact angles and ultrasonic frequencies were investigated respectively.?2?The mechanism of chemical mechanical polishing and material removal model were analyzed.On this basis,the properties of fluid and the dynamic changes of abrasive in ultrasonic assisted chemical mechanical polishing were simulated and studied from the point of view of fluid action,which has been less considered but has great influence.The cavitation phenomenon of fluid under ultrasonic vibration was introduced and its effect on pressure and temperature change of fluid were quantitatively analyzed,simultaneously,the sonochemical effect was also analyzed.Based on the VOF and DPM model in multiphase flow,during ultrasonic assisted chemical mechanical polishing and conventional chemical mechanical polishing,the performance parameters,such as pressure,temperature and flow rate of the fluid were investigated by the CFD simulation reported in literature,which can provide guidance for subsequent experiments and result analysis.?3?The composition and dynamic principle of the conventional single-sided free abrasive lapping process system were analyzed.On the basis of this,in order to realize the normal ultrasonic vibration of the specimen and the in-situ continuous adjustment of the machining load,the ultrasonic vibration assisted experimental setup was designed and manufactured,which can be used for subsequent test research.The structure design and working principle of the key functional components were introduced,where the positioning method of the specimen and the operation process of the device were analyzed.The design process of stepped horn was analyzed,the correctness of which was verified by modal and harmonic response analysis,finally the manufactured horn was tested by impedance analysis and laser Doppler vibrometer,and the results meet the experimental requirements.In addition,a simplified equivalent model of ultrasonic vibration assembly was established,base on which the force analysis of the specimen was analyzed.?4?Based on the self-designed and manufactured device,the ultrasonic assisted free abrasive lapping for single crystal SiC wafer was studied.The experimental arrange(L27(313))for lapping pressure,speed of lapping plate,abrasive particle size and ultrasonic power is designed by orthogonal method,and the test results were analyzed by signal-to-noise ratio?SNR?and grey correlation method.Taking the material removal rate,roughness Ra and Rmax as single index respectively,the influence of process parameters and the optimum process combination were investigated,simultaneously the comprehensive properties was analyzed by taking the material removal rate,roughness Ra and Rmax together into consideration,and the influence of process parameters and their optimal combination were determined.Furthermore,the lapping effect of large amplitude on single crystal SiC is analyzed by expanding the range of ultrasonic amplitude.The material removal rate and surface generation characteristics of single crystal SiC wafer by different lapping plates under ultrasonic and non-ultrasonic auxiliary process were investigated.?5?Based on the self-designed and manufactured device,the ultrasonic assisted chemical mechanical polishing for single crystal SiC wafer was studied.Firstly,the corrosion test of single crystal SiC was carried out with polishing solution for chemical mechanical polishing.By comparing the surface composition of single crystal SiC after conventional corrosion and ultrasonic vibrated corrosion,it was determined that ultrasonic assistance could promote the chemical corrosion in chemical mechanical polishing.After that,through the combined test of lapping and chemical mechanical polishing,the results showed that ultrasonic-assisted lapping combined with ultrasonic-assisted chemical mechanical polishing can improve the final polishing effect of single crystal SiC wafer.Furthermore,experiments for SiC wafer were conducted according to the combinational process assisted by ultrasonic vibration,and the results showed that it can achieve the thinning and smooth undamaged surface with a higher efficiency. |
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