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当微结构特征尺寸与入射光波长相近或更小时,必须采用矢量衍射理论来分析不同电磁场分量在器件表面中的相互耦合作用,该研究利用FDTD solution软件对金刚石红外透过率进行定量计算,该软件基于时域有限差分法(FDTD),对空间电磁场进行时域分析。多晶CVD金刚石的复折射率来自Refractive. Index数据库,基于这些数据建立金刚石材料模型,仿真区域在 x、 y方向使用周期性边界条件(Periodic), z方向使用完美匹配层(Perfectly Matched Layer,PML),设置光源为平面波,沿 z方向无角度入射。
金刚石微结构的制备过程可分为三个阶段:
(1)制备硅衬底。利用设计好的掩膜版,对硅片进行反应耦合等离子体刻蚀(ICP),采取Bosch工艺控制刻蚀侧壁角度和深度,刻蚀过程如 表1所示,步骤1和步骤2交替循环。
表 1 刻蚀参数
Table 1. Etching parameters
Steps Gas flow of SF 6/sccm Gas flow of C 4F 8/sccm HF Power/W ICP Power/W Temperature/℃ Pressure/mTorr(1 Torr=133.32 Pa) Time/s 1 0 100 10 700 20 40 6 2 100 0 25 700 20 30 7 (2)生长金刚石。分别以微结构硅片和平面硅片为衬底生长金刚石,衬底尺寸均为10 mm×10 mm,厚度为0.5 mm。生长前依次用丙酮、酒精、去离子水对衬底进行超声清洗。然后将衬底在200 nm金刚石粉-酒精混合液中超声处理,引入晶粒提高形核密度。MPCVD工艺生长金刚石,衬底温度保持770~780 ℃,生长功率1 650 W,腔压11.5 kPa,氢气流通量300 sccm,开始阶段甲烷流通量15 sccm高温形核,然后9 sccm生长,生长时间145 h。
(3)后期处理。对样品生长面进行研磨、抛光,使表面粗糙度降至20 nm以下,然后用3:1的氢氟酸与硝酸混合液溶解硅,得到微结构金刚石薄膜。
通过扫描电子显微镜(美国, QUANTA FEG 250)观察表面微结构硅衬底和金刚石形貌,评价金刚石的复制生长效果,显微共焦激光拉曼光谱仪(英国Renishaw, inVia-Reflex)检测薄膜的金刚石质量,傅里叶红外光谱仪(美国varian, Excalibur 3100)检测薄膜的红外光学透过率。
Study on infrared anti-reflection performance of diamond film with surface microstructure
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摘要: CVD金刚石是一种性能优异的红外光学窗口材料,但其在红外波段的理论透过率仅能实现约71%。通过表面亚波长结构设计可进一步增强CVD金刚石膜的光学透过性能。该研究首先通过理论模拟,建立了金刚石微结构特征与光学增透之间的定量关系。基于此为指导,探讨了在具有微结构硅片表面,采用MPCVD方法复制生长出具有微结构的金刚石自支撑光学级薄膜,用于提升金刚石膜在红外波段的透过率。采用扫描电镜(SEM)观察了原始硅片和金刚石表面及微结构形貌,通过拉曼散射光谱评估了金刚石的生长质量及形核层影响,采用红外光谱仪测试了金刚石红外透过率。结果显示,单面构筑微结构后,金刚石膜在8~12 μm波段的透过率可从70%提升至76%,说明表面微结构能显著提升金刚石膜的光学透过性能。非金刚石形核层以及表面微结构的完整性不足可能是导致实验结果与理论模拟结果具有一定偏差的主要原因。Abstract: CVD diamond is an excellent material for infrared optical window, but its theoretical transmittance in the infrared band can only achieve about 71%. The optical transmission performance of the CVD diamond film can be improved by the surface sub-wavelength structure design. In this study, the quantitative relationship between diamond microstructure characteristics and optical antireflection was established through theoretical simulation. According to the theory guidance, the CVD diamond film with surface microstructure was fabricated by replicating the Si substrate through MPCVD method to improve the transmittance of diamond in the infrared band. Scanning electron microscope (SEM) was used to observe the surface and microstructure of the original silicon wafer and diamond. The growth layer quality and the nucleation layer quality of diamond were both evaluated by Raman scattering spectrum. Infrared spectrometer was used to test the infrared transmittance of diamond film. The results show that after constructing the microstructure on one side, the transmittance of the diamond film in the 8-12 μm band can be increased from 70% to 76%, which means the surface microstructure can significantly improve the optical transmission performance of diamond film. The non-diamond nucleation layer and the insufficient integrity of surface microstructure may be the main reason for the gap between the experimental results and the theoretical simulation results.
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Key words:
- diamond film /
- sub-wavelength structure /
- infrared antireflection /
- simulation
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表 1 刻蚀参数
Table 1. Etching parameters
Steps Gas flow of SF 6/sccm Gas flow of C 4F 8/sccm HF Power/W ICP Power/W Temperature/℃ Pressure/mTorr(1 Torr=133.32 Pa) Time/s 1 0 100 10 700 20 40 6 2 100 0 25 700 20 30 7 -
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