飛秒雷射切割與聚晶鑽石輪刀研削之複合式製程技術應用於碳化矽晶粒分割研究

Abstract

碳化矽晶圓屬寬能隙及低阻值材料，具耐高溫與耐高壓特性，適於5G和電動載具的高功率及高頻晶片製作。但碳化矽鍵結強度大硬度高，晶粒分割相當困難，本研究提出一種以飛秒雷射搭配聚晶鑽石輪刀切割的「複合式晶粒分割製程技術」，用以對碳化矽晶圓進行晶粒分割。實驗先以線切割放電加工對聚晶鑽石輪刀進行削正，再以線上放電削銳技術進行銳化，銳化的輪刀不但具高同心精度，且能於輪刀表面創造出高密度鑽石切刃與屑袋，深寬比可達14:1(刃長500 μm及刃寬35 μm)，非常適宜難削材料的窄溝槽成形研削(Grinding)。在晶粒分割方面，實驗先以飛秒雷射對碳化矽晶圓進行切割，在雷射作用下，極短脈衝能量迫使少許的碳化矽材料被極速剝蝕，鍵結破壞，強度與硬度因而大幅降低，故鑽石輪刀能以低的研削力在相同的雷射路徑上研削被破壞的碳化矽。實驗結果發現，複合式製程技術可使鑽石輪刀於晶粒分割時，載台電流由0.9 A降至0.45 A，陣列溝槽的垂直壁面具高平整度與高一致性，溝崩比從11.7降至4.1，即使輪刀填塞，藉由線上放電削銳技術也能快速恢復。證實此複合式晶粒分割製程技術不但具高精度及高可控性，能有效降低碳化矽的研削阻抗與溝崩比，延長輪刀使用壽命，特別適宜高強度與高硬度的第三代半導體的晶粒分割，深具商業化價值。Silicon carbide (SiC) has wide band gap and low resistance, which is suitable for the high-power chips of 5G and electrical vehicle application due to the electrical properties as well as the tolerance of high-temperature and -pressure. However, it’s difficult to dice the SiC wafer because of the strong chemical bonds and high hardness. In view of this, a hybrid dicing process technology which combining femtosecond laser cutting with polycrystalline diamond (PCD) wheel tool grinding to dice dies of SiC wafer is proposed in this study. The PCD wheel tool is first trued by the w-EDM and then dressed by the in-situ EDM. The sharpened wheel tool not only has high concentric accuracy, but also can create high density diamond cutting edges and chip pockets on the surface of the wheel tool, and the aspect ratio can reach up to 14:1 (edge length 500 μm and edge width 35 μm), which is very suitable for narrow groove grinding of hard-to-cut materials. The femtosecond laser is first employed to cut SiC wafer in the experiment. The very short pulse energy forces a small amount of SiC material to be ablated extremely fast from the CNC path, making a bond-breaking, thus significantly reducing the local strength and hardness of SiC. Hence, the PCD wheel tool can grind the damaged SiC on the same laser path with a low grinding force. Experimental results show that the stage current as the grinding resistance can be reduced from 0.9 A to 0.45 A by the hybrid dicing process technology when grinding. The vertical wall surface of the microgrooves array has high-flatness and -consistency, and the kerf chipping ratio (KCR) is reduced from 11.7 to 4.1. Even if the wheel tool is dulling, it can be recovered quickly by in-situ EDM. It is proved that the proposed technology not only has high precision and controllability, but also can effectively reduce the grinding resistance and the KCR of SiC, and extend the service life of the wheel tool, which is especially suitable for the dies dicing for the high-strength and -hardness third-generation semiconductors, and is of great commercial value.