高反射率Mo/B_4C多层膜设计及制备

Design and fabrication of high reflectivity Mo/B_4C multilayer mirrors

运用遗传算法优化设计了Mo/B4C多层膜结构。入射光入射角度取10°时,设计的理想多层膜膜对数为150,周期为3.59 nm,Gamma值(Mo膜厚与周期的比值)为0.41,峰值反射率为33.29%。采用恒功率模式直流磁控溅射方法制作Mo/B4C多层膜。通过在Mo/B4C多层膜与基底之间增加15 nm厚的Cr粘附层,提高多层膜与基底的粘附力。另外,还采用调整多层膜Gamma值的方法减小其内应力,调整后多层膜结构周期为3.59 nm,Mo膜厚1.97 nm,B4C膜厚1.62 nm,峰值反射率26.34%。制备了膜对数为150的Mo/B4C膜并测量了其反射率,在波长7.03 nm处,Mo/B4C多层膜的近正入射反射率为21.0%。最后对测量结果进行了拟合,拟合得到Mo/B4C多层膜的周期为3.60 nm,Gamma值0.60,界面粗糙度为0.30 nm。

Mo/B4C multilayer is a suitable material combination for high reflectivity mirrors in the soft X-ray wavelength region of 6.7-8.0 nm. The structures of the ideal Mo/B4C multilayer mirrors optimized based on the General Algorithm method （GA） are that the bilayers number is 150, the period is 3.59 nm, the ratio of Mo film thickness to period is 0.41 for the fixed incident of 10°. The refleetivity of such a Mo/B4C multilayer is 33.29% at first order Bragg refleetivity peak. Mo/B4C multilayers were fabricated with a DC magnetron sputtering coater. All these multilayers were deposited on super-polished silicon substrates, of which the area is about 20 mm×30 mm. The surfaeerms roughness of the Si substrate is less than 0.3 nm. The purity of the target materials Mo, B4C and Cr is respectively 99.95%, 99.50% and 99.95%. Cr-based layer with 15 nm thickness was inserted between of the multilayer and the substrate to improve the adhesive ability between them. In order to reduce the stress of the Mo/ B4 C multilayer, the ratio of multilayer was changed from 0.41 to 0.55. Accordingly, the final parameters of Mo/B4C multilayer were 3.59 nm period, 1. 97 nm Mo thickness, 1. 62 nm B4C thiehness, and the reflectivity of such multilayers is 26. 34%. Through the two improvements introduced above, the Mo/B4C multilayer mirrors with 150 layer pairs were fabricated. The fitted roughness factors is 0.30 nm. The measured refleetivity of Mo/B4C multilayer-mirror is about 21.0% at 7.03 nm for 10° incidence. The theoretical analysis of the measurement data presents that the fabricated Mo/B4C multiplayer is with 3.60 nm period, 0.60 ratio,and the roughness factor is 0.30 nm.