超音波振動輔助磨削碳化矽晶圓之研究

Abstract

單晶碳化矽(Silicon Carbide, SiC)具有耐高溫，寬能帶的特性，因此作為第三代半導體材料備受市場青睞。然而單晶碳化矽具有高硬度的特性，使加工時容易遭遇許多困難，包含材料去除率低，工具損耗率高等問題，這些加工限制皆會導致最終成本的提高。 過去已有研究證明超音波輔助磨削(UAG)對磨削加工有正面的影響，目前也已經有部份廠商將超音波輔助應用於精磨削碳化矽，然而尚未有廠商將超音波輔助應用於粗磨削領域。過去研究也鮮少提及砂輪硬度對於超音波效果的影響，對於超音波輔助時材料的去除機制也說明得不夠完整。本研究利用超音波輔助磨削單晶碳化矽，比較與傳統磨削(CG)的效果差異，同時以不同硬度之砂輪配合超音波輔助，探討超音波輔助效果隨時間變化之趨勢，並解釋UAG時的材料去除機制，期望研究成果能有助於將超音波應用於粗磨削碳化矽製程。 實驗結果顯示UAG相比於CG，無論是砂輪消耗量，垂直方向力或扭矩皆有所下降，且材料移除率也明顯上升。表面粗糙度方面所有組別的表面粗糙度皆隨實驗進行逐漸上升，初期兩種砂輪在UAG時所磨出的表面粗糙度皆高於傳統磨削，到了後期由於CG的增長幅度大於UAG，因此表面粗糙度大於UAG。而比較不同硬度的砂輪可發現UAG時硬砂輪的效果優於普通砂輪。 UAG時砂輪出現高頻，穩定的週期性振動，此垂直方向的運動使磨粒更好的刺入材料表面，減少磨粒與材料表面的磨擦(Rubbing)，達到磨削力降低以及提高材料去除率的效果。隨著實驗的進行由於磨粒因磨削而變鈍，使砂輪與工件之間的接觸面積增加，平均應力下降而使超音波輔助效果隨加工進行逐漸下降。此外週期性的振動也使磨粒於材料表面反覆進行力的負載與卸載，材料次表面會產生密集的側向裂紋使材料表層的硬度降低，有利於加工。

Monocrystalline silicon carbide (SiC) has been favored as a third-generation semiconductor material because of its high-temperature resistance and wide energy bandwidth. However, the high hardness of Silicon Carbide (SiC) makes it easy to encounter a lot of difficulties in machining, including low material removal rate and high tool wear rate, and these machining constraints will lead to an increase in the final cost. In the past, studies have proved that ultrasonic-assisted grinding (UAG) has a positive effect on the grinding process, some manufacturers have already applied UAG to the fine grinding of SiC, however, no manufacturer has yet to apply UAG to the coarse grinding. The effect of grinding wheel hardness on the ultrasonic effect is rarely mentioned in the past studies, and the mechanism of material removal during ultrasonic assisted grinding is not fully explained. In this study, we tried UAG for monocrystalline silicon carbide, compared the effect difference with conventional grinding (CG), also different hardness of grinding wheels were used to investigate the trend of the effect of UAG with the change of time. The material removal mechanism during UAG were also explained, in the hope that the results of this study can be helpful for the application of ultrasonic waves in the rough grinding of silicon carbide process. The experimental results show that compared with the CG, the UAG shows a decrease in wheel wear, vertical force and torque, and a significant increase in material removal rate. In terms of surface roughness, the surface roughness of all groups increased gradually with the experiment. Initially the surface roughness of both grinding wheels in UAG was higher than that of CG. At a later stage, the surface roughness of CG was higher than that of UAG due to a larger increase in CG than that of UAG. A comparison of different hardnesses of the wheels showed that the hard grinding wheels was better than that of the conventional grinding wheels during UAG. When UAG, the grinding wheel has high frequency, stable periodic vibration, so that the abrasive can easily pierce through the material surface, reduce the friction between abrasive and material , achieving the effect of reducing the grinding force and improve the material removal rate. As the experiment progresses, the contact area between the grinding wheel and the workpiece increases due to the passivation of the abrasive by grinding, and the average stress decreases, so that the ultrasonic assisting effect gradually decreases with the processing. In addition, the periodic vibration also causes the abrasive grains to load and unload repeatedly on the surface of the material, causeing the material’s subsurface to produce dense lateral cracks, resulting the decrease of the hardness of the material surface, which is conducive to machining.