| 摘要: | 碳化矽(Silicon carbide, SiC)為第三代半導體的關鍵材料，相較於目前使用量最高的矽晶圓，有更優異物理特性與化學穩定性，其中更高的熱導率與更寬能隙，使它在高功率與高頻元件上更具優勢。然而，SiC晶圓的製造成本卻一直居高不下，主要挑戰來自: (1) 前段長晶製程的困難耗時，因此不易獲得質精、量大的晶錠；以及，(2)後段加工製程的修邊、切片、研磨、與拋光等過程耗時長且高耗能。此兩瓶頸導致SiC晶圓的製作成本一直居高不下，也是SiC晶圓製造亟需突破的關鍵。晶圓薄型化與晶圓更高效的切、磨、拋技術研發，是SiC晶圓製造業者積極應對高材料成本的研發方向。 其中超快雷射隱形切片(Stealth slicing)技術可切割更薄晶圓且同時微小化切割面的表面材料損傷，薄型化可大幅提高材料的使用率，低表面損傷則可有效地降低後續研磨、拋光的時間與耗材消耗。
本研究以飛秒雷射探討N型4H-SiC的隱形切割技術，發現經由改變雷射的處理參數與雷射光在晶圓內部的聚焦深度，可在晶圓內部形成單一改質或雙改質切層，且兩切層的寬度與形貌並不相同。實驗發現，在脈衝能量(Pulse energy, Ep) 8.56 µJ 與脈衝重疊率(Pulse overlap, OP) 70.8%時，可以獲得穩定且均勻的單一改質切層，其位置，如預期地，在雷射光聚焦平面；而在相同的脈衝能量下，若將脈衝重疊率提高到OP = 85.4%時，則首度發現除了可在可在原聚焦平面獲得改質的切層外，也會在其上方(沿入射光的方向)形成第二個的改質切層，也就是，同時形成雙改質切層。實驗也發現，此一現象可重複出現，且發生在特定的雷射處理參數範圍內；此外，本研究也發現，經由改變雷射參數，此兩改質切層間的距離也會有變動。
對於此第二改質層的形成，本研究提出其形成機理，認為是因為在脈衝頻率提升的情況下，在其入射光的路徑上因材料熱累積溫升所導致非線性材料吸收率變化所引起，因此，當非焦平面位置的雷射能量密度達到SiC改質的閾值後，即可形成第二個改質層，故其成因並非是「雷射燈絲現象」(Laser filamentation)，而本研究也經由實驗，驗證此一機理。據此機理，當改變雷射脈衝能量時，可變化此雙改質切層之間的距離，本研究也實驗驗證當雷射脈衝能量增加時，雙改質切層之間的距離即隨之增加。經由横切面的研磨與蝕刻，本研究進一步觀察此兩改質層的結構，在本研究目前的參數下，第二改質層雖不在焦平面上，但其被改質範圍卻大於在焦平面的第一改質層。
工程應用上，此同時形成雙質切層技術，除了加倍SiC切片效率外，也可直接達成厚度小於100 µm的SiC晶圓切片。
;Silicon carbide (SiC) is a key material for third-generation semiconductors. Compared to the most widely used silicon wafers, SiC exhibits superior physical properties and chemical stability. Its higher thermal conductivity and wider bandgap provide it advantages in high-power and high-frequency devices. However, the manufacturing cost of SiC wafers has remained high due to two main challenges: (1) the difficulty and time-consuming nature of the initial SiC crystal growth process, resulting in limited availability of high-quality and large-volume ingots; and (2) the time-consuming and energy-intensive processes in the later stages of edge trimming, slicing, grinding, and polishing. These bottlenecks have kept the production costs of SiC wafers consistently high, posing a critical challenge for the SiC wafer manufacturing industry. The development of technologies for wafer thinning and more efficient cutting, grinding, and polishing processes has become a proactive research direction for SiC wafer manufacturers in response to the high material costs. Among these, the ultrafast laser stealth slicing technology enables the cutting of thinner wafers while minimizing surface material damage. Thinning wafers significantly increase material utilization, and reduced surface damage effectively lowers the time and consumables required for subsequent grinding and polishing processes.
Experimental findings indicate that with a pulse energy (E_p) of 8.56 µJ and a pulse overlap (OP) of 70.8%, a stable and uniform single modified layer is achievable, forming at the expected laser focus plane. Remarkably, under the same pulse energy, an increase in pulse overlap to OP = 85.4% led to the discovery that not only a single modified layer was formed at the original focus plane but also a second modified layer above it (along the direction of incident light), resulting in the simultaneous creation of a double modified layer. The study further notes that this phenomenon can be replicated within specific ranges of laser processing parameters. Moreover, alterations to the laser parameters were observed to cause variations in the distance between these two modified layers.
Regarding the formation of this second modified layer, this study proposes its formation mechanism, suggesting that it is due to the increase in pulse frequency. The non-linear change in absorption coefficient caused by the cumulative temperature rise along the path of incident light is believed to be the cause. Therefore, when the laser energy density at the non-focal plane position reaches the threshold for SiC modification, the second modified layer can be formed. Consequently, its origin is different from the "Laser filamentation" phenomenon, and this mechanism has been experimentally validated in this study. Based on this mechanism, variations in the pulse energy of the laser can change the distance between these double modified layers. The study also experimentally verifies that as the laser pulse energy increases, the distance between these two layers also increases. Through grinding and etching of the cross-sectional surface, this study further observes the structure of these two modified layers. Under the current studied parameters, although the second modified layer is not at the focal plane, its modified range is greater than that of the first modified layer at the focal plane.
In practical applications, the technology of simultaneously forming double modified layers not only doubles the efficiency of SiC slicing but also allows for the direct achievement of SiC wafer slices with a thickness of less than 100 µm. |
