基于液相AFM及干涉频谱法的多孔氧化铝薄膜在线测量系统研究

介绍了多孔氧化铝薄膜(PA)厚度在线测量系统的工作原理、系统结构及独特性能。在白光(宽光谱)照射下,薄膜的上表面和下表面反射的两路光线发生干涉,产生了携带薄膜光学厚度信息的反射光谱。同时,利用液相原子力显微镜(AFM)实时获得的PA膜的表面形貌信息,根据膜系统的Maxwell-Garnette有效介电常数理论,经相干势近似计算得到薄膜的有效折射率,进而得到此时PA膜的物理厚度。使用该系统对PA膜氧化制备过程进行了在线扫描和膜厚测量试验,成功的获得了PA样品的实时表面形貌图像,得到样品的孔隙率和有效折射率。并根据样品反射光谱,利用反射干涉频谱法计算得到氧化150和180min时,PA膜厚分别为5.35和6.25μm。本系统具有测量简便、实时性好、无损及测量精确的特点,在实时测量和监控膜厚的同时可获得样品的表面形貌、孔隙率、有效折射率等信息。

Study of In-Situ Measurement System for Porous Alumina Film Based on AFM and Reflectometric Interference Spectroscopy

An in-situ measurement system for porous alumina (PA) film based on atomic force microscope (AFM) in liquid and reflectometric interference spectroscopy (RIFS) was developed. The present article briefly discusses the principle and structure of the system, and introduces its unique characteristic. The system consists of probe unit, XY scanner, Z-piezo feedback system, computer and software, fiber optic spectrometer, anodization control circuitry etc. When a white light beam illuminates the surface of the film, the reflective light beams at the front and back side of the layer are coherent, and lead to periodical amplifications and extinction in the reflective spectrum with the information of the optical thickness of the film. A fiber optic spectrometer was applied in the system which input the refractive spectrum into the computer by which the optical thickness of the film was calculated. Meanwhile according to the surface topography of PA films by AFM in liquid, the effective refractive index was calculated based on Maxwell-Garnett theory and coherent potential approximation (CPA). So the thickness of PA films could be gained at last. To checkout the feasibility and stability of the system, the real-time scanning and thickness measurement experiments were done during anodization of Al sheets in oxalic acid aqueous solution. In the experiment, the authors used 25 mm diameter aluminum (Al) sheets with 99.999% purity and 0.4 mm thickness as the anode, and graphite rod as the cathode. The pretreatment-cleaned Al sheets were anodized in an aqueous solution of 0.5 mol x L(-1) oxalic acid at the constant temperature (20 +/- 0.2) degrees C with 20 mA x cm(-2) anodization electronic current density. Real-time AFM images of PA film were successfully obtained during anodization. The pore-ratios of Al sheet were 7.81% and 13.83% at oxidizing time 150 min and 180 min respectively. Correspondingly, the effective indexes were calculated to be 1.62 and 1.60, respectively. Combining the reflective spectrum gained by the fiber optic spectrometer, the wave number differences between the two adjacent extrema were 57.69 mm(-1) and 50.00 mm(-1). Based on reflectometric interference spectroscopy, the thickness of PA films was found to be 5.35 and 6.25 microm with oxidizing time 150 min and 180 min respectively. The system has characteristics of convenient manipulation, non-destructive and high-resolution in-situ measurement. And the surface morphology, pore-ratio and effective refractive index can also be gained at the same time.