Study On The Controllable Fabrication Of Silicon-based X-ray Refractive Lenses By Metal Assisted Chemical Etching

The development of techniques of focusing X-rays has occupied physicists for more than a century.In recent years,X-ray refractive lenses have also made great progress.The traditional fabrication of X-ray refractive lenses based on silicon is usually accomplished by dry etching,such as deep reactive ion etching（DRIE）and LIGA（Lithographie,Galvanoformung,and Abformung）process,these methods require a high vacuum environment and hence still suffers from the problems such as expensive equipment,requiring high vacuum environment,complicated processing steps,and limited etch depth.As a new wet etching method,metal assisted chemical etching（MACE）is a recently developed anisotropic electrochemical corrosion method,which has also been widely studied owing to its features of being simpler and more cost-effective.In order to realize the simple and controllable manufacture of X-ray refractive lenses,this thesis combines the MACE with the preparation of X-ray refractive lenses for the first time,and mainly focuses on optimizing the etching parameters of MACE.The main work of this thesis is as follows:1.The conditions that affect the morphology of the sidewall during the silicon processing were investigated,including:the doping type of wafer,the type of catalytic metal,the thickness of the catalytic metal film,and the concentration ratio of hydrofluoric acid to hydrogen peroxide in the etchant（ρ=[HF]?（[HF]+[H₂O₂]）,[HF]and[H₂O₂]represent the concentrations of HF and H₂O₂,respectively）.The etched silicon structures with vertical sidewalls and smooth floors were obtained with the optimized parameters.2.The factors that affect the structural integrity and etching uniformity during the processing of silicon were explored,including:etching temperature,stirring speed,and the ethanol content in the etching solution.The experimental results showed that by adding ethanol to the etching solution,the etching rate can be effectively reduced,resulting in the etched structures with great integrity and uniformity.3.According to the bulk micromachining parameters optimized in the previous experiments,representative X-ray refractive lenses designed by ourselves was fabricated.The experimental results show that samples with5 nm Ti layer via magnetron sputtering as the adhesive layer and followed a16 nm Au film evaporated by electron beam as the catalytic metal film were etched in an etching solution withρ=0.57 and ethanol concentration of 12.5M,the obtained compound refractive lenses（CRLs）and saw-tooth lenses have vertical sidewalls and smooth floors with nanometer roughness,along with good integrity and uniformity.4.In response to the phenomenon that the integrity and uniformity were improved by the addition of ethanol,a new explanation was proposed that the lower solubility of fluorosilicic acid in ethanol than that in water can suppress the progress of the etching reaction,which thereby slowed down the etch rate and relieve the damage of the catalytic metal film from the reaction product（the released hydrogen）,and eventually led to the improvement of the etching uniformity.The low-cost,efficient,and controllable preparation method proposed in this paper provides an insight for further studies on the manufacture of Micro-Electro-Mechanical System（MEMS）devices such as X-ray refractive lenses,and provides theoretical guidance for the morphology control of silicon bulk micromachining using MACE.