强各向异性Be薄膜的晶粒细化和应力弛豫机制研究

The beryllium film with the isotropy characteristics and low residual stress is required in the physics experiment for inertial fusion energy (IFE). During the Be film growth, the Be atoms array in the form of hcp structure, and the crystal grains grow along c-axis orientation. The Be film is mainly characterized by a large size columnar grain, and displays a strong anisotropy characteristics. In addition, the residual stress in Be films produces unavoidablly. It will lead to distortion and cracking of film. These disadvantages reduce the mechanical property and ablation characteristics for Be films, as well as destroy the formation of favourable shock-wave face. The Be film is restricted to use in IFE. The objective of this project is to achieve grain size refinement for the strong anisotropy Be film. On the basis of the classical nucleation theory, we try to control critical nuclear size and prevent from the grain extension growth via coupling the new methods of a strong magnetic field and an ultra-sound field, and plan also to reduce the formation of film defects and dislocation during the non-equilibrium growth by in-situ anneal processing or heating-up processing. Further, we investigate the dynamics process of Be particles, such as coagulation, nucleation and growth. Some relationships between grain size and outer field intensity would be established. The film residual stress would be measured by X-ray diffraction using sin2ψ method, and then the stress relaxation mechanism was explored.

在聚变能（IFE）实验研究中，需要用到各向同性且低应力的Be薄膜材料。Be薄膜在生长过程中原子以hcp结构排列，晶粒c轴取向，以粗大柱状晶为主，薄膜呈现强各向异性。另外，在薄膜制备过程中应力产生在所难免，它会引起薄膜变形和开裂，这些降低了Be薄膜力学性能和烧蚀特性，破坏了良好的冲击波阵面形成，限制其在IFE中的应用。本项目基于经典形核理论，拟通过施加强磁场和超声场来降低临界形核尺寸和阻止晶粒外延生长，实现薄膜晶粒细化。结合薄膜原位退火和分级热处理技术，借助热扩散来减少薄膜在非平衡生长过程中形成的缺陷和位错，进而降低薄膜残余应力。深入研究Be粒子在基体上凝结、成核、长大的动力学过程，建立薄膜晶粒尺寸随外场强度的函数关系，采用XRD-sin2ψ法表征薄膜残余应力，并揭示薄膜应力弛豫机制.

强各向异性Be薄膜因其原子序数低，X射线不透明性低等，常作为惯性约束聚变和高能量密度物的烧蚀靶材。在本项目三年的支持下，主要开展了Be基薄膜生长动力学、形态演化、晶粒细化、相组成、表面粗糙度、残余应力和电学性能等研究，并获得了以下重要结果和新的认识：.蒸镀Be薄膜的生长速率随温度呈指数增长，而溅射薄膜随溅射功率呈线性增长。采用蒸镀法与溅射法制备Be薄膜的沉积速率能够任意调节，蒸发温度1100℃对应磁控溅射47 W的沉积速率。不管制备方法，基片材质和工艺参数等，制备的Be薄膜均由hcp结构的α-Be相组成。两者制备的Be薄膜微观形态演化丰富多彩，在较低沉积速率下，两者均以细小的等轴晶生长为主。.热蒸发温度在1050℃-1150℃，对应Si基片上的沉积速率为1.1-25.3 nm/min。而磁控溅射功率在20-240 W，它的沉积速率为1.5-30.3 nm/min。沉积速率1.4-25 nm/min，蒸镀薄膜的晶粒尺寸从25 nm增加到270 nm，而磁控溅射法的可达340 nm，磁控溅射比蒸镀法制备的薄膜晶粒尺寸略大。对比热蒸发和磁控溅射制备的铍薄膜，膜厚在130 nm-2570 nm范围，两者表面粗糙度相当。.采用四探针法测量了溅射Be靶材的电阻4.3×10-8 Ω.m，它比Be薄膜的电阻小30倍。主要原因是Be薄膜密度比块体材料低，仅为块体的94％-98％。因此，薄膜内存在缺陷和孔洞，增加了电子的散射。在超声场和强磁场耦合下，分别实现了Be薄膜的晶粒细化。蒸镀Be薄膜晶粒尺寸126 nm-130 nm，在超声场场作用，晶粒尺寸减小至60 nm，降低了2倍。而在磁场作用下，晶粒尺寸为18 nm，降低了7倍。显然，磁场晶粒细化比超声晶粒细化更明显。.当溅射气压在0.4-1.2 Pa时，Be薄膜的残余应力在69-559 MPa。衬底温度在60℃-410℃，Be薄膜应力在-153 MPa至425 MPa之间。显然，采取合适的溅射气压和衬底温度（溅射气压在~0.4 Pa，衬底温度130℃-200℃），能够实现Be薄膜的应力弛豫。