Silicon Laser Processing with Bursts

飞秒激光精密微加工目前广泛应用于商用光源发射波长约800- 1000nm。然而，体积内和背面的修改仅限于介电材料，如玻璃，在这个波长范围内是透明的。硅作为微电子领域应用最广泛的半导体材料，是难以获得的。SILABUS项目解决了极具挑战性的问题，即利用超短激光脉冲实现硅的内部和背面修改。超短的相互作用时间可以获得极高的精度，因为相互作用被限制在非常小的体积内，并且避免了有害的副作用或周围材料的损坏。作为窄隙半导体（带隙为1.12 eV），硅仅在1100nm以上的波长处透明。然而，由于硅的固有特性，如何将这种技术实现到硅上并不明显。具体来说，硅的非线性折射率比熔融石英高两个数量级。因此，超短激光脉冲在硅中的传播容易发生非线性畸变，并且强度在低于材料永久改性阈值的水平上饱和。为了解决这一基本问题，我们建议使用飞秒激光，发射波长在1300-2000 nm左右，并以一种新的模式发射，即GHz-burst模式。这种方法将允许将传送的能量分配到许多子脉冲中，这些子脉冲以GHz的突发内重复率相互跟随，允许以累积和可控的方式在焦点处产生永久修改，从而避免上述有害的非线性效应。

Femtosecond laser precision micro-machining is nowadays widely applied using commercially available sources emitting at wavelengths around 800-1000 nm. However, in-volume and back-side modifications are restricted to dielectrics like glasses which are transparent at this wavelength range. Silicon, as the most utilized semiconductor material in microelectronics, is not accessible. The SILABUS project addresses the very challenging issue of achieving internal and backside modifications in silicon with ultrashort laser pulses. The ultrashort interaction time allows for obtaining an extremely high precision as the interaction is confined in a very small volume, and avoiding detrimental side-effects or damage of the surrounding material. As a narrow-gap semiconductor (band gap of 1.12 eV), silicon is only transparent at wavelengths above 1100 nm. However, it is not evident how to implement this technique to silicon due to its intrinsic properties. Specifically, the nonlinear refractive index of silicon is two orders of magnitude higher than the one of fused silica for instance. Therefore, ultrashort laser pulse propagation in silicon is prone to nonlinear distortions and the intensity is saturated at a level below the threshold of permanent modification of the material. We propose to resolve this fundamental problem by using a femtosecond laser emitting around 1300-2000 nm in a novel regime which is the GHz-burst mode. This approach will permit to distribute the delivered energy into many sub-pulses that are following each other at a GHz intra-burst repetition rate allowing for the production of permanent modifications at the focal point in an accumulative and controlled way, and thus, avoiding the aforementioned detrimental nonlinear effects.