Design And Fabrication Of Diffractive Optics Elements For Infrared And Terahertz Radiation

The wet etching technology is an important method for fabrication diffraction micro-optics elements (DOEs). A large number of complex fine microstructures can be fabricated over the surface of silicon wafer through the method according to the erosion characteristics of several special silicon crystal orientations in KOH solution and common microelectronics techniques. Compared with other approaches, the diffractive elements with fine morphology and complex micro-nano-structures can be fabricated according to the wet etching technology studied in this thesis, which only consists of single mask photolithography and dual-step wet KOH etching. The method demonstrates some merits, such as simple procedures, easily controlling of microstructural and technological parameters, and low cost.In this thesis, the amplitude modulation DOE used in infrared (IR) and terahertz (THz) wavelength range, are designed and fabricated through the wet KOH etching processes. Both Mid-IR (3-5μm) and Far-IR (8-14μm) in common infrared range, and then three wavelengths of 118.83μm and 122.4μm and 158.51μm in terahertz range, are selected as target wavelength to achieve the design and fabrication of needed micro-nano-structures and devices. The whole technological flow consists of four key steps including: calculation phase distribution map, designing photomask, dual-step wet etching, and performance measurements.Firstly, based on the Fresnel theory, the phase distribution patterns are calculated by G-S algorithm. Secondly, the distribution map is transferred into photomask through combining the erosion characteristics of special silicon crystal orientation in KOH solution. The dual-step wet etching process is the core part of this study. The anisotropic wet KOH etching and the automatic stop of KOH erosion operation are the theoretical and technological foundermental. During dual-step process, the first-step wet etching is performed to form inverted pyramid microstructures with needed heights and further complex step-shaped phase microstructures with desired heights is formed after the second-step wet etching. Finally, the surface presentation of the fabricated DOEs, such as surface roughness, step-shaped outline, wavefront, and imaging properties, are measured. Experimental results demonstrate that the fabricated DOEs have some remarkable characters, for instance, the surface roughness reaching the level of optics mirror, the step-shaped microstructures being obvious, and having ideal phase and amplitude modulation efficiency.