Multistep neutral density filter by ultra-precisely controlling the thickness of nano-Ni80Cr20 film

• High precision neutral density filters by ultra-precisely controlling the thickness. • Atomic-layer etching is achieved by bombarding films with low energy ion beams. • Showed the different etching condition of Hall and Kauffman ion source. Ni80Cr20 alloy film shows a good neutral transmission in visible light. A large area and high precision neutral density filter with multiple steps made of Ni80Cr20 is presented in this work, which proposes one manufacturing method for controlling film thickness. The quartz crystal sensor in the coating machine has a thickness uncertainty larger than 5%, which leads to the low accuracy of optical density value of the filter. To solve this problem, we bombarded the membrane surface with low energy ion beams when the optical density of the filter was slightly larger than we wanted. The absolute error of optical density could then be controlled within 0.01, and the relative error is ±2%, a result which is much better than those that accompany existing commercial neutral density filters. Finally, a neutral density filters with 18 steps is manufactured, which can used as a component of a spectral sensitivity spectrometer.