1.School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China 2.Science and Technology on Space Microwave Laboratory, China Academy of Space Technology (Xi’an), Xi’an 710100, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. U1537211, 61501364).
Received Date:16 August 2018
Accepted Date:10 January 2019
Available Online:01 March 2019
Published Online:20 March 2019
Abstract:Multipactor is a frequent discharging phenomenon for space high-power microwave components, and this detrimental effect is mainly induced by secondary electron emission (SEE) and electron resonance in vacuum. Plenty of researches have verified that suppressing SEE is an efficient approach to mitigate the multipactor. Therefore, low SEE yield surfaces are urgently needed for mitigating the multipactor in the field of space science. In the past few decades, a number of technics have been developed to acquire low SEE yield surfaces, including surface coating, surfaces roughening, depositing coessential nanostructure, etc. Laser etching has been partly reported to be an advisable way to construct micro- or nano-structure on some materials’ surfaces, and able to further suppress the SEE yield. Whereas, employing laser etching to obtain the SEE yield reduction on gold coated surfaces is rarely investigated. In this work, by using the laser etching technic, we fabricate four micro hole arrays and three orthogonal groove arrays with various porosities and aspect ratios, and we also characterize their three-dimensional and accurate two-dimensional morphologies. In addition, we investigate the dependence of SEE yield on surface morphology. Experimental results indicate that the laser etched microstructures can effectively suppress the SEE yield from gold coated surfaces, and the suppression levels on SEE yield of these samples are superior to those of many other low SEE yield technics. Furthermore, experiments reveal that the ability to suppress the SEE yield is positively related to the porosity and aspect ratio, as well as that the porosity influences SEE yield more strongly than the aspect ratio does. To theoretically verify the experimental phenomena, we utilize the Monte Carlo method combining with the SEE phenomenological model and the electron trajectory tracking algorithm, to simulate the SEE characteristics of the fabricated microstructures. And the simulation results can qualitatively explain the experimental phenomena. This work digs out an advisable method to sharply reduce the SEE yield from gold coated surfaces by laser etching, which is of considerable importance for exploiting the low SEE yield surface engineering in space microwave systems, and for improving the performance of the space microwave components with gold coated surface. Keywords:laser etching/ microstructure/ secondary electron emission/ secondary roughness
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2.1.激光刻蚀制备微阵列结构
实验中使用波长为1064 nm的脉冲式红外光纤激光器(GSS-Fiber-20, 西安高斯激光)对铝合金样品表面进行扫描刻蚀. 激光器最大刻蚀功率为20 W, 最小束斑直径约为$10\;{\text{μ}}\rm{m}$, 最大可加工区域面积为110 mm × 110 mm, 激光焦距约为150 mm, 刻蚀过程中工作环境为大气环境. 实验选用的刻蚀样品为铝合金样片(样品尺寸: 20 mm × 10 mm, 厚度为1 mm). 实验中通过改变每个刻蚀单元内刻蚀面的形状及尺寸以达到控制样品表面刻蚀区域的目的, 通过改变激光功率大小及扫描刻蚀次数控制刻蚀区域的深度. 如图1(a)和图1(c)所示分别为两种刻蚀图形样式: 圆孔微阵列和正交沟槽微阵列, 图1(b)和图1(d)分别给出了两种微阵列中单个刻蚀单元的图样和尺寸标注. 实验中通过控制单点加工次数和激光功率, 制备了4组如图1(a)所示的圆形孔洞阵列(#1—#4样品); 通过控制刻蚀区域的特征尺寸, 制备了3组如图1(c)所示的正交沟槽阵列(#5—#7样品). 7组样品详细加工参数及各刻蚀图形尺寸在表1中列出. 表1中理论孔隙率(记为A)定义如下: 单元面积内, 刻蚀区域投影面积与单元投影总面积之比, 对于圆柱微孔阵列, 其理论孔隙率可由(1)式计算得到: 图 1 刻蚀图样 (a)微孔阵列; (b)微孔阵列刻蚀单元; (c)正交沟槽阵列; (d)正交沟槽阵列刻蚀单元 Figure1. Etching patterns: (a) Micro hole array; (b) etching unit of the micro hole array; (c) orthogonal groove array; (d) etching unit of the orthogonal groove array
样品编号
#1
#2
#3
#4
#5
#6
#7
激光功率/W
16
12
10
6
12
12
12
$d/{\text{μ}}\rm{m}$
150
150
150
150
150
100
50
$b/{\text{μ}}\rm{m}$
50
50
50
50
50
100
150
$l/{\text{μ}}\rm{m}$
200
200
200
200
200
200
200
扫描次数
400
100
25
5
250
250
250
理论孔隙率A/%
44.18
44.18
44.18
44.18
43.75
75.00
93.75
表1各样品刻蚀单元内图形尺寸及激光刻蚀参数 Table1.Sizes of the etching patterns and the detailed etching parameters