Bi靶激光等离子体极紫外光源以及碎屑辐射特性研究

Characteristics of Extreme Ultraviolet and Debris Emission From Laser Produced Bi Plasma

激光等离子体极紫外光源具有体积小、稳定性高和输出波长可调节等优势,在极紫外光刻领域发挥着重要的作用。Bi靶激光等离子体极紫外光源在波长9~17 nm范围内具有较宽的光谱,可应用于制造极紫外光刻机过程中所需的极紫外计量学领域。利用平像场光谱仪和法拉第杯对Bi靶激光等离子体极紫外光源以及离子碎屑辐射特性进行了实验研究。在单脉冲激光打靶条件下,实验中观察到Bi靶激光等离子极紫外光谱在波长12.3 nm处出现了一个明显的凹陷,其对应着Si L-edge的吸收,是Bi元素光谱的固有属性。相应地在波长为11.8和12.5 nm位置处产生了两个宽带的辐射峰。研究了两波长光谱特性以及辐射强度随激光功率密度的变化。结果表明,在改变聚焦光斑大小实现不同激光功率密度(0.7×10^(10)~3.1×10^(10) W·cm^(-2))过程中,当功率密度为2.0×10^(10) W·cm^(-2)时两波长处的光辐射最强,其原因归结为Bi靶极紫外光辐射强度受激光能量用于支撑等离子膨胀的损失和极紫外光被等离子体再吸收之间的平衡制约所致。在改变激光能量实现不同激光功率密度过程中,由于烧蚀材料和产生两波长所需高阶离子随着功率密度的增加而增加,增强了两波长处的光辐射。进一步,研究了双脉冲激光对Bi靶极紫外光谱辐射特性影响,实验发现双脉冲打靶下原来在单脉冲打靶时出现在波长13~14 nm范围内的凹陷消失。最后,对单脉冲激光作用Bi靶产生极紫外光源碎屑角分布进行了测量。结果表明,当探测方向从靶面法线方向移动到沿着靶面方向上的过程中,探测到Bi离子动能依次减小,并且离子动能随激光脉冲能量降低而呈线性减小。此项研究有望为我国在研制极紫外光刻机过程所需的计量学领域提供技术支持和打下夯实的基础。

Laser produce plasma extreme ultraviolet(EUV)source,which has the advantages of small size,high stability and adjustable output wavelength,plays a significant role in applying EUV lithography.Laser produces plasma Bi EUV source has a wide spectrum in the wavelength range of 9~17 nm,which can be used to apply extreme ultraviolet metrology in the development of extreme ultraviolet lithography.Therefore,EUV emission and debris characteristics from laser-produced Bi plasma were carried out.When the 1064 nm pulse laser irradiated the Bi target,a natural dip displays at 12.3 nm in the EUV spectrum,corresponding to the L-edge absorption in silicon.Meanwhile,two strong peak emissions are located at 11.8 and 12.5 nm,respectively.Firstly,we studied the emission characteristics and intensity of the spectrum near the 11.8 and 12.5 nm dependence on laser power density.When the laser power density is adjusted by changing the focus spot size by fixing the laser energy,the emission intensity of two peaks increases first and then decreases with an increase in the laser power density.The maximum emission intensity of two peaks was formed when the laser power density of 2.0×10^(10) W·cm^(-2).This is attributed to the final output EUV emission is determined by the balance of the laser energy loss used to support plasma expansion and reabsorption of the EUV emission by the plasma.When the laser power density is adjusted by changing laser energy by fixing the focus spot size,the emission increases with an increase of laser power density due to the ablation material and high stage ions increases.Secondly,we studied the effect of dual pulse on the emission intensity of the 11.8 and 12.5 nm peaks.The experiment results show that the emission intensity of two peaks increases gradually when the laser energy increases from 20~140 mJ.Moreover,the intensity decreases when the laser energy larger than 140 mJ due to the EUV emission being absorbed by the thick plasma at a larger plasma density.In addition,it is found that the dip generated in the spectrum at a 13~14 nm wavelength with a single pulse laser disappeared when using the dual pulse method.Finally,we measured the angular distributions of ions emission from a 1064 nm laser-produced plasma.The results indicated that the kinetic energy of Bi ions decreases when the detection direction moves from the normal direction of the target surface to the direction along the target surface due to the plasma preferential expansion perpendicular to the target surface.Moreover,the kinetic energy of Bi ions decreases linearly with the decrease of laser pulse energy.This research is expected to provide technical support and lay a solid foundation for the metrology field needed in the development of EUV lithography.