A novel optical structure of numerical aperture increasing lens (NAIL) for resolution improvement in backside failure analysis

As the development of VLSI and scaling down & multi-metal-layer of semiconductor devices, there was the obstacle for failure analysis (FA) from front side of device. So, FA from backside was developed in microelectronics yield in recent years. As is known to all, we could capture clearer infrared (IR) image from backside as Si substrate was thinner. But if we needed higher resolution image with conventional optical objective lens, we must introduced NAIL or shorter wavelength light to improve numerical aperture (NA) in objective space. In this paper, we proposed one novel optical structure of NAIL to enlarge aperture angle in objective space to obtain larger NA value and higher resolution. We introduced the principle of NAIL, designed our optical structure, and analyzed its characteristic. Its advantages were that (1) the variable refractive index of liquid material and liquid made light scattering decreased, (2) common solid body material-glass which made fabrication process simply due to its larger size. So, we believed the novel optical structure was beneficial to our FA from backside.