Silicon Laser Processing with Bursts

飞秒激光精密微加工目前广泛应用于使用波长在800-1000 nm左右的商用光源。然而，体积和背面修改仅限于在此波长范围内透明的玻璃等介电材料。硅作为微电子中利用率最高的半导体材料，是不可获得的。SILABUS项目解决了使用超短激光脉冲实现硅的内部和背面修改这一非常具有挑战性的问题。超短的相互作用时间允许获得极高的精度，因为相互作用被限制在非常小的体积内，并避免了对周围材料的有害副作用或损害。硅是一种窄禁带半导体(禁带宽度为1.12 eV)，只有在1100 nm以上的波长下才是透明的。然而，由于硅的固有特性，如何在硅上实现这一技术并不明显。具体地说，硅的非线性折射率比熔融二氧化硅高两个数量级。因此，超短激光脉冲在硅中的传输容易出现非线性失真，并且在低于材料永久修改阈值的水平上强度饱和。为了解决这一根本问题，我们提出了一种新的工作模式--GHz-Burst模式，利用1300-2000 nm的飞秒激光来解决这一问题。该方法将允许将所传递的能量分配到以GHz脉冲串内重复速率彼此跟随的许多子脉冲中，从而允许以累积和受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.